That "free" wire is actually a bit of a millstone around your neck in your situation
The problem with your "free" 4/0 wire and quadruplex (4/0-4/0-4/0?-4) is that it's somewhat too large for the job at hand. You see, devices like breakers, switches, and panels can't accept arbitrarily large wires for simple reasons of physical space, both for lugs large enough to fit large wires and more importantly for the space required to bend large, stiff wires to get where they need to go. This is a particular issue for you because you need to insert a visible blade disconnect somewhere in your solar feeder to satisfy your utility.
This is required by your utility's interconnect rules by way of incorporating Texas PUC Rule 25.212, and practically means that you need to fit a non-fused safety switch to provide that visible-blade disconnect. However, a 100A safety switch doesn't have enough room for 4/0 wire in it; one can special order some safety switches with oversized lugs and enclosures, but that's going to be a costly proposition for a one-off project. You could use a 200A safety switch instead of the 100A switch, but that's a cost adder as well, and it just gets worse when you add in the additional cost of collar lugs, subfeed blocks (vs. branch breakers), beefed-up subpanels (200A vs 125A), and heavier-duty service equipment.
As to your main panel...
Your next problem is finding a suitable main disconnecting means, given the sheer amount of solar power you're harvesting (64A of it with both inverters running flat-out). Unfortunately, while a solar-ready meter-main that provides a dedicated solar input breaker is in some ways the ideal solution as it doesn't cut into busbar ampacity, very few meter-mains of that type that I am familiar with can handle an 80A solar input breaker. Worse yet, your utility only apparently approves of a small selection of meter sockets, with no meter-main devices among them.
The good news, though, is that that the other normal solution for this problem, namely a "mobile home service" panel with feed-thru lugs running off to the house, will work for your situation, because there's a 200A main breaker at the house that'll protect the feeder to the house from inadvertently getting overloaded. Since you're planning for future upgrades to the solar system, though, you'll want a 225A busbar on your main panel as well as the ability to take a 175A breaker for the solar infeed.
The first criteria rules out most mobile home feeder panels, unfortunately; the GE/ABB TM822RCUFL is the only made-for-purpose mobile home panel panel with a 225A interior. The second is also a problem, as GE branch breakers upwards of 125A are not well-known parts. You might be able to find a THQMV175WL through a supplyhouse, but applying it would be tricky at best due to the splicing required, so we'll need an alternative solution.
Fortunately, Siemens provides us with one in the form of the PNW1224L1225C. While this comes configured as a main-lug panel, we can fit it with a MBK200A or 225A kit to convert it to main breaker and also add a ECLK2225 subfeed block + ECLK3N extra neutral lug to serve in place of the feed-through lugs. Finally, while the QN2175(R) and Q2175B are both special-order parts, most supply houses that deal in Siemens stuff should be able to order them for you, answering the question of a 175A breaker when paired with another ECLK3 neutral lug opposite the subfeed block.
Note that either way, you're limited to a total of 50A for other loads (pumphouse, security fence/gate operation, serviceperson's receptacle) and a CLSURGE for a service surge protector by the NEC 705.12(B)(2)(3) point 3 limits, and your main panel will need the appropriate premade warning label required by 705.12(B)(2)(3) point 3, as well. (The surge protector is a new 2020 NEC requirement, found in 230.67; normally, I'd recommend an external Type 2 SPD, but those require a breaker to connect to the panel, and that breaker would push you over the busbar limits even though the SPD draws no load.) You could use a 30/20 or 35/15 triplex breaker (two single poles and a two-pole in a two-space package) if you want support for feeding the security loads from the main panel, or a 50A two-pole if you want the pumphouse subpanel to feed those instead.
Alternative main panel plans 1: tapping the meter
If the plan of using a mobile-home "trailer panel" or field-fabricated equivalent as the sole service equipment isn't workable, there are a couple of alternatives. First off, since your utility largely uses Milbank meter sockets, you can look at using a set of Milbank K5022-INT tap lugs on the load side of the base to pull off a set of 1/0 Al conductors for the solar switch, which goes on the opposite side of the meter base from your main panel. Note that the solar switch needs to be a fusible switch in this case as it's now a piece of service entrance hardware; 125A RK5 fuses are reasonable (and reasonably cheap) for this, and will accommodate your existing solar setup and proposed partial upgrade.
Since we don't want the wellhouse to get shut off by the solar disconnect, though, that leaves us with two options for powering it. Either you can use a 200A mobile-home panel or equivalent (such as the Homeline one you're looking at) to provide breaker space for both a whole-service surge protector and the branch/feeder breakers (wellhouse, fence/gate hardware), or you could use a special set of lugs on the line-side of the solar switch to tap power there.
The latter, of course, depends on what you get for said switch: Square-D provides a flexible option for their 200A switches in the AL20DTF, but it's not listed, so its applicability depends on your AHJ, while Siemens switches can accept a control tap kit, but that only provides a tab terminal, not a full screw lug, so it limits how much power you can run to the wellhouse. With both switch options, you'll also need to provide another service disconnecting means at the rack, dedicated to the wellhouse, although it can be as simple as a fusible A/C pullout disconnect, with a pair of conduits connecting it to the solar-switch.
The former isn't without its caveats either, though: you can't run the wellhouse feeder back through the service conduits as NEC 230.7 prohibits service and non-service conductors from sharing a raceway, so you'll need to fit a "runaround" conduit from the main panel to the solar-switch. This can be either a rigid conduit run from the back of the panel to the back of the switch using LBs or pull-elbows, or a liquidtight flexible conduit run from the bottom of the panel to the bottom of the switch around the service riser using 90° fittings at each end.
With either of these, though, you'll be constrained by the need to run a minimum of 2 neutrals in your solar and house conduits, as you can't cross-connect neutrals that originate in two different service disconnect enclosures. This also means you'll need to use care when wiring 120V loads from the backup power system; even if not required, using different colors, tape flags, or so on to denote backup-power/solar neutrals is a good plan in this circumstance.
And all that grounding business...
As far as grounding goes, you'll need two 8' rods driven 6-8' apart at the meter rack, with a 6AWG copper grounding electrode conductor going from both rods up into the meter socket and terminating there, as per PEC's service loop drawings. Fortunately, you don't need to worry about providing a grounding wire in your conduits, as the quadruplex has you covered on that front.
At the inverter rack, the grounding wire makes a pitstop at the solar combiner panel before continuing onward alongside the pumphouse feeder. In addition, you'll need another pair of ground rods and matching grounding electrode conductor at the rack, headed on up to the grounding bar in said solar combiner panel. At the pumphouse, though, you can use the well casing as a grounding electrode, provided it's made of metal of course; if it's a plastic casing, then you'll need to drive ground rods and connect them up to the grounding bar on the pumphouse panel.
Of course, all the bonding screws in all the subpanels need to be pulled out, and the house will need its own grounding electrode system (made rods, metal water line, Ufer/concrete-encased electrode), tied to the grounding bar in the house's panel. Finally, you will need to run a neutral out with the feeder to the solar system as your inverters don't support 240V-only connections to center-grounded systems, and probably should continue that neutral out to the pumphouse as well in order to provide 120V there without the bulk of a local step-down transformer. (Sharing a neutral between multiple feeders is permitted by NEC 215.4(A), although that won't work if they're on the load side of different service disconnects -- you'll need 2 neutrals and thus 9 wires minimum in the solar/pumphouse conduit in that case.)
