Staying out of trouble where neutral meets ground
The first problem with your idea of using an "el cheapo" interlock kit with an arbitrary portable generator is that while the interlock deals with making sure the utility's hots don't get backfed, it does nothing to help you switch the neutral between the utility and the generator. While you can't get in trouble for backfeeding the utility this way, you can cause a bunch of consternation for yourself, as your average portable generator is going to ship with its neutral bonded to its frame and ground connections so that it can be safely and legally used for portable (i.e. jobsite) power. This means that if you plug it into a system that doesn't switch the neutral, you'll have your generator's bond connected in parallel with the neutral-ground bond in your main panel, which can trip GFCIs on the generator and cause wayward neutral current to flow on normally nonenergized grounding wires.
Avoiding this requires the use of a transfer switch or panel that switches the neutral wire to the standby loads between the "utility" and "generator" side neutral wires in addition to switching the hots. While these aren't as common as switching setups that use a solid neutral, they do exist in a few different variations. The one we'll be using here is the XRK0603DR from Reliance Controls; while not nearly as cheap as an interlock, it provides a fairly convenient package in an outdoor-rated enclosure, although you'll have to fit the matching flanged inlet (L1430F for a 30A inlet to match your 30A generator) yourself. (If you get a bigger generator, you can use an XRK0605DR and a matching LL530F inlet instead. Note that with this setup you will need a California Standard 50A plug on your generator cord to go with your inlet, as there is no NEMA L14-50 configuration.)
With either of these, you'll need a label on the transfer switch saying it's for connection of a separately derived (bonded neutral) system, as per NEC 702.7(C). Note that NEC 110.21(B) requires you to use a preprinted label/decal for this instead of something handwritten; your local sign shop should be able to find or make you something suitable that you can stick on the transfer switch. This is atop the NEC 702.7(A) requirement that the service entrance equipment be labeled to denote where the standby source is (which can be satisfied by a note in the panel directory, since it isn't a caution, warning, or danger sign or label), and in your situation, the NEC 702.7(B) requirement that the service equipment bear a preprinted warning label about how disconnecting the GEC there would undo the generator's connection to earth.
Sidebar: what if you do get a floating neutral generator after all?
If you do somehow wind up with a floating neutral genset that you can't reconfigure to have a bonded neutral, this does not require a change of transfer hardware, luckily. What you can do instead is "fill in" the missing bonding jumper using a tap connector off the generator neutral wire within the transfer switch and a piece of 8AWG or 6AWG copper connected from that neutral wire to the transfer switch grounding bar. You'll need to use the correct NEC 702.7(C) label if you do this, though!
Interlocked and bypassed out
The other problem with your interlock proposal is that Class 320 single-main-breaker meter-panels don't support interlocks best I can tell, and their multiple-main-breaker counterparts are being rendered obsolete by the 2020 NEC's restrictions on the use of "rule of six" disconnecting arrangements. Fortunately for us, all this is mooted by the neutral-switching issue, so we can simply go ahead and use a single-breaker Class 320 meter-panel for this, although we do need to make sure we have one that meets your utility's specifications, as your utility, like most of the Washington State PUDs and many other West Coast utilities, requires EUSERC compliant metering hardware.
Fortunately, the Siemens MC2442B1400SD you are considering fits that bill; this should be available through local electrical supply houses without too much trouble (the SDS variant has a test block bypass, so check for that too). If you can't find that particular part, Milbank makes a suitable substitute known as the M404-UG-LC(-BS for a test block bypass), or you can use an Eaton U404430MC(C for a bypass) or HP402442 for that matter, although the last part is not available with a bypass feature. All of these of these supply most of the knockouts we need, although you'll need to punch a couple of KOs in the right side yourself no matter which of these you pick.
A sidenote on architecture before we start wiring things up...
Note that I only talked above about a single transfer switch; this is acceptable in this application since Accessory Dwelling Units or other contrivances that may require separate metering are not a concern based on what you have said. It does mean that we're running utility and standby feeders to the garage, though, vs. running utility and generator feeders. This plays nicely with the idea of putting in battery storage at the house, and isn't a barrier to putting the batteries in the garage, either, as running a second feeder in the standby feeder conduit to the garage can be done without impacting its load carrying capacity too heavily if you play your cards right.
Note also that all of the aforementioned meter-panels are compatible only with underground service, not overhead; fortunately, your service is underground, like most Class 320 services, rendering this a non-issue for you. Finally, we'll need to put in a whole-house surge protector for this service in order to meet the new 2020 NEC 230.67 requirement for surge protection on residential services.
Now that we have all that out of the way...
Now that we have all that out of the way, we can start discussing how this all fits together. First up is the meter-panel, sitting atop the 3" PVC riser from the utility. A 2" and a 1.5" PVC conduit run out of the meter-panel to the garage/shop space in a 24" deep trench, along with a second 1.5" conduit laid in the same trench but capped off at its stubups so that you don't need to dig anything up if you want to run fiber out to the shop.
In the meter-panel, we start by fitting a 60A breaker in the top left for the utility side of the transfer switch and a 50A breaker in the top right for the surge protector. Below these two, we fit two 200A breakers, one each for the house and garage/shop feeders; the rest of the spaces in this panel are left open, though. Once we've done that, we make a 1/2" KO in the right side of the panel in line with the 50A breaker and fit a Type 2 SPD there, using a pair of sealing locknuts to attach it so that water can't get in through a hole that'd let it drip onto live buswork.
With all that out of the way, we then can concern ourselves with connecting the transfer switch. First off, we'll need a 1.25" by 12" rigid nipple; this goes into a field-punched 1.25" KO on the right side of the meter main that aligns with the matching KO on the left side of the transfer switch chosen. Once the boxes are mounted and the nipple's installed, we can then proceed to wire it up with 6AWG copper from the utility terminals to the 60A breaker in the meter-main and the generator terminals to the inlet's connections. It then gets fitted with another 50A or 60A breaker for the feed to the house and a 20A, 2-pole breaker for the standby feed to the garage/shed. This latter feeder is wired with 4 12AWG wires headed back through the nipple to the 1.5" conduit coming off the main panel to the garage.
Moving onto the larger feeders, a four-wire feeder gets run from the transfer switch outside to a main lug subpanel inside for the actual standby loads in the house (you'll see why when we start talking about solar and batteries) using either a 6/3 cable of some sort, or 4 6AWG wires in whatever flavor of conduit you pick. (If you use NM for this, you'll be limited to 50A; you'll need to use conduit of some flavor, armored cable, or type SE cable in order to get 60A out of this, or go to a 4/3 NM cable for that matter.) With that out of the way, we then can wire up the two big 200A feeders; the feeder going into the house gets run out the back of the meter-main with a 250kcmil-250kcmil-250kcmil-3/0 SER cable as 2" conduit won't fit into a 2x4 stud wall even with stud shoes on the holes, while the feeder to the garage is run using individual 250kcmil Al XHHW-2 wires inside the 2" PVC conduit from before alongside a 6AWG copper ground wire (green or bare).
Tying all this down to Planet Earth
Now, we can consider the grounding electrode systems this all requires. At the house, you'll need to run a 4AWG copper wire from the meter-panel, via a suitable box connector (Arlington GC50 or equivalent), to the main house grounding electrode system, and fit an Intersystem Bonding Termination bridge to that copper wire for that matter to provide a place to land communications (phone, satellite-TV, and so on) grounding wires. Note that this is 4AWG, and not the 2AWG that'd otherwise be called for by Table 250.66, since your grounding electrode is an Ufer and thus falls under NEC 250.66(B) instead.
Moving onto the garage, you'll need to use more of that 6AWG copper wire from earlier to connect the grounding bar of the garage's primary (200A) panel to a pair of 8' ground rods driven at least 8' apart. The garage standby feeder, then, can be landed on a second, smaller main breaker panel, using a wirenut to splice the neutrals in the latter case, and a length of 6AWG copper to connect the grounding bar on the standby disconnect to the garage's grounding electrode system no matter what you use for a disconnect.
As to that solar + storage thing...
Now that all the basics are taken care of, we can start concerning ourselves with solar and storage matters. First off, we start with straight grid-tied solar, as that's the easiest/simplest case. While tying your solar system in at the main panel is the correct thing to do in this situation as it simplifies the figuring considerably, your existing plans don't account for the limits of the busbars in the main panel. NEC 705.12(B)(2) point (3)(b) limits you to a total feed-in of 480A, split between the main breaker and 125% of the solar breaker, and requires the solar breaker to be at the opposite end of the panel from the main breaker as well. Running the numbers, this means you are limited to a maximum of 64A of solar on an 80A breaker, which translates into just over 15kW of solar generation.
Adding storage changes this situation quite considerably; since a storage (multimode) inverter can form a grid all on its own, it can run in conjunction with grid-tied solar inverters (what's called "AC coupling"), a fuel-burning generator, or even solar inverters and a generator at the same time, at least as long as your batteries aren't full. However, they rely on a very different power transfer configuration to accomplish their goals; in particular, instead of transferring a load between two independent sources, they have a single throw switch between the inverter-load connection and the power input, so that they can export power to the grid when a utility grid is present.
However, most multimode (solar + storage) inverters listed for use in the US do not transfer or otherwise switch the neutral wire, nor do the interface-devices (Backup Gateway or equivalent) used with AC-coupled energy storage systems. This means we can't use any internal input-to-input transfer they provide with a portable generator. However, that works out OK in the end here since we already have a switching neutral transfer panel fitted to handle that job.
The end result is that you wind up with the battery/inverter setup wherever you wish it (at the garage is fine), with a transferred (utility or generator) feeder from the transfer panel running to the multimode inverter or transfer device (Backup Gateway, in Powerwall terms) via a maintenance disconnecting means. The output of the multimode inverter or transfer device then runs over to an AC distribution panel with feeders from that to the house and garage/shop standby panels. That panel can also have an interlock on it to provide a bypass function so that standby power can be provided to loads while the battery inverter is inoperable, although that may not be advisable in some configurations. The solar inverters also get re-routed to tie into that AC distribution panel, or a panel downstream of that AC distribution panel, in this setup, as that's simpler than trying to DC-couple the solar system with its attendant power electronics and DC switching/protection issues.
Note that since you're putting the storage in the garage for the last setup, you'll have to replace that existing 12AWG feeder and 20A breaker for the garage with a 60A breaker and a 6AWG copper or 4AWG aluminum feeder when you put the storage system in. In addition, you'll have to put a 60A breaker and another 6AWG Cu or 4AWG Al feeder to feed standby power from the AC distribution panel at the garage back to the house. This can run in the same conduit as the transfer-switch feed to the garage, though, as NEC 310.15(B) permits you to base the Table 310.15(B)(3)(a) adjustments off of the 90°C ampacity column when using 90°C rated conductors such as today's THHN/THWN-2, and for 4 current-carrying 6AWG wires (as neither the neutrals nor the ground count), 80% of 75A gives us exactly 60A. Last but not least, you'll need to move the 60A breaker feeding the transfer panel down to the bottom of the main breaker box in order to use 705.12(B)(2) point (3)(b) as described above.
Note, by the way, that this limits you to 11.5kW of DC coupled storage with up to 11.5kW of solar behind it, or a combination of 11.5kW of solar and AC coupled storage. If you want to go up to the full 15kW possible with your main panel while using the solar while the power's out, you'll need to use an XRK1003DR (instead of the XRK0603DR) for the transfer panel, along with 80A breakers instead of the 60A breakers, 2AWG Al wire instead of the 6AWG Cu or 4AWG Al in the standby-feeder conduit between the house and garage, and a 2-2-2-4 Al SER cable from the transfer panel to the house standby panel instead of the 6AWG Cu cable called for above.