What does "Ampacity Dwelling" mean?
It means nothing at all, because it's coming from a wire salesman's website. They don't write the electrical code and they don't interpret it. Wire sites are chock full of lies. One site I like tells me #6 is good for 105A. (I know where they got that number, but it's disgraceful to publish it.)
Only the Electrical Code matters. If it's #10 or smaller you go to 240.4(D). Anything larger, you're at 310.15(B)(16), "The Big Table". You look up your cable type in the top row, that tells you the thermal limit of the cable. You're also limited by the thermal limit of connectors, so even if the wire has a 90C limit, you're pushed down to 75C or even 60C. I've never seen a case where someone got away with using 90C numbers in residential. You wouldn't want to anyway if you're talking about upsizing for voltage drop. NEC is concerned with not burning your house down, not your pocketbook.
Anyway, once you're in typically on feeder this size the 75C column, you work down to your target ampacity, 200A. Oh, look at that. 3/0 copper or 250 kcmil aluminum. That was easy. Rounding up or down didn't even come up!
Now, if your feeder size is anywhere near your service capacity, you make one more pass, using NEC 310.15(B)(7). Your service wires (which carry all the current downstream of the meter get a favorable 83% derate. And this is saying that your feeders never have to be bigger than your service wires (because that would be stupid). So take your 200A service size, multiply by .83, and get 166A. Back to table 310.15(B)(16) and we find that 2/0 copper @ 175A is legal as a 200A service wire, so no feeder downstream of that needs to be larger than that.
So I guess the numbers on that site aren't wrong, but the point is, you cannot trust them as they are not a proper source of data.
Regardless, I strongly advise Buy the wire LAST. Seriously. It's the least returnable thing in your project. Buy it only after you have finished learning, and haha, joke is, you never will be! So buy it as late as possible, so you will benefit from as much information as possible.
"Is my 150A panel good for 300A @ 120V?"
Yup. Kinda. It's actually NOT good for 300A. It's good for two banks of 150A. Here's what I mean. Suppose you have a machine that draws 32A actual and you must derate it 125% to 40A, as you usually must do. How many of those "40A" provisions can you put on 150A @ 240V? Seven of them (280A)? NO! Each 150A bank can support only three (120A) each. So six total.
That said, that only applies to loads you PLAN to run simultaneously by design such as banks of grow lights in a shed. General loads in your house, such as your 2 circuits per bedroom, are counted a totally different way, that allows for the fact that you won't be running everything at the same time. There's a whole section in Code on doing load calculations for a whole house, and that decides your minimum service size.
An interesting side effect of those rules is each kitchen and bathroom receptacle circuit adds 1500 VA (think watts) to your load calculation, but other circuits don't. So your additional bedroom circuits are "free".
Is 3/0 SER right for 200A to a sub?
Depends. If your service is 200A it's larger than need be. If your service is larger than 200A it's the minimum allowable size. Voltage drop is not a factor over such a short distance. SER is not a choice I would make, since inside conduit it's hard to pull, and without conduit, it is not upgradeable or replaceable.
What I would do: Panels.
First, let's talk about your ambitions. They are quite correct. "I want to do this once, and I want to do it right" - yes, that's how we work! Being picky about what's installed: with ya there. Not trusting anyone else to get this right - oh yeah. "I don't trust anyone else to do it the "right" way" - absolutely - so true. "Most people you hire will do things to the minimum code requirement." - yup, can't agree more. NEC minimums are for slumlords. Yet a lot of people follow it like it's the Bible - someone recently reported having a 5-bathroom house with all bathroom receptacles sharing 1 circuit, technically legal in NEC, but man, I hope that guy enjoyed the latté they bought with the "big saving" on wires.
Picking up on the idea of "200A now, more later", I'd take full advantage of the way "400A" (Class 320) service is currently actually implemented in North America. 400A breakers are prohibitive. So they install TWO 200A main panels - just run-of-the-mill, common-as-dirt, $130 panels from the box store. Most 400A meter-mains are specifically designed for this configuration, and are lugged for dual 250 kcmil wires (that's 200A in aluminum). Since they're not rated for paralleling, this is the only way they can be used.
However, NEC 2020 requires the main disconnect must be outside. It's by far the cheapest to integrate "main disconnect" and "main breaker", so now, "Meter-Mains" (meter and main breaker in one enclosure) are the option of choice. How is that handled in 400A? By having meter-mains with dual 200A main breakers.
They also sell "meter-main-panel" that combine all of them in one. These are a bad idea except for what are called "farm panels" which provide 8 breaker spaces and then thru-lugs to carry power onward to your primary panel (the one with most of your loads in it). I like them. The 400A versions of these are still the same deal, two 200A main breakers, but the internal panel is only fed by one of those breakers.
The second 200A breaker isn't even provided - that's so you can fit up a 100 or 200A breaker depending on if you have 300A or 400A service.
So for me in your shoes, there are two ways to go with this, both involve installing a 200A "panel inside the house" for now. On one path, you simply install a 200A meter-main and be ready to trash it later, but you keep your "panel in the house" and it becomes one of your two 200A panels. On the other path, you install a 400A meter-main now, and simply do not populate the second 200A breaker. When you take the upgrade, you populate the other breaker and feed that to a 2nd "panel in the house". In fact you could wire up this second panel today even; just feed it from the first panel.
Of course the panels would be enormous. Running out of breaker spaces is the most unnecessary crisis we deal with on here. Breaker spaces are dirt cheap. Honestly, what gets my goat is the endless parade of people who come on here, they spen, and that shouldn't be a problem if you're sympatico with copper feeder, but yet, so many of the novices we see who are out of spaces chintzed out on the panel and spent on copper wire. Just don't do that okay? :)
What I'd do: Conduit heck yeah
You know the builder who wouldn't deal with you because you wanted better than Cat 5? Are you kidding me!!?? Fine, you get Cat 7, what happens 5 years from now when you never see your kids - they're over at the neighbor's house watching his holographic TV because your ratty old Cat 7 can't support it? Honestly if you built your house 20 years ago, you'd have cat 3 buried in your walls and no 4K TV for you. Enough. Chasing trends is a loser's game. The only constant is that houses live much longer than tech.
What we do know, for sure, is that signals will travel over something long and thin - might be fiber, might be coaxial, might be spaghetti. But it'll be long and thin. So I would install conduit and call it a day. Not only does it allow you to change wire, it allows you to add wires.
Conduit is more initial work, but obviously that's not a deal-breaker for someone who wants the best.
Further, conduit really lends itself to the novice. And here I'll throw in a pitch for EMT metal conduit. Because it assembles like an Erector Set and if you mess up (which is understandable as you are learning), you can dismantle it and salvage all the fittings and most of the pipe.
Yes. EMT for telecomm. Least, better shielding, plus the Erector Set thing.
I haven't even talked about running AC Power in conduit, but that works even better. I would definitely do it for runs such as panel-to-workshop or panel-to-garage where you are likely to want to add or change things.
The best way to do that is run THHN individual wires (i.e. a black wire and a white wire, loose in the pipe).
The gotcha with AC power in conduit is you have circuit limits. There's at least 4 Code sections that interlock to mean in US 120/240V power, "4 circuits per conduit if the breakers are 30A or less". (There's a corner case involving a 28-30A non-continuous load with 4 circuits in a pipe, but never mind that). For >30A, you need to do the math with 310.15(B)(3)(a) "multiple circuit derate", but it eases the pain that you're allowed to derate off the 90C column. (which is why it's a non-factor for up to 4 <30A circuits.)
I myself prefer multiple 1/2" conduits anyway; it's easier to work with, and the sweep radius is smaller (easier to build around). You can have up to 4 sweeps between access points, and the access points must remain accessible forever.
After all, you pull the wires last, so you can build the conduit and then leave the actual pulling to the electrician. Many of them object to doing only part of a job, but don't mind so much if the part you did was competent conduit.
What I'd do: Many circuits.
2 circuits per kitchen is one of those slumlord bare minimums. The fact is, any common heating appliance is 1500W, because that is the maximum UL will approve for the US market. And if you're following the bouncing ball, you know that's 12.5 amps i.e. you can't run 2 of them on the same circuit. That means 1 heat appliance per kitchen circuit, and suddenly 2 isn't really enough.
And bathrooms! Holy smoke! 2 circuits per bathroom is again not excessive, at least for the master suite. Bathrooms sharing a circuit is right out!
Now let's talk about bedrooms - and pause to ask yourself: "Where did the idea of "1 circuit per bedroom" even come from in the first place? I sure don't know, it sounds like it's something that somebody made up. Well, whoever they are, they clearly never wired a house. Do they really mean to circumnavigate a bedroom with wire, sharing each wall with a different circuit? Ridiculous!
I say Run the wires down a wall, and serve rooms on both sides. If you run 1 circuit per wall, that means each bedroom has access to 4 circuits! (In practice you bend that a little bit because of physical layout, but the general concept holds. YOu can put 60-80A in any room, (though not in every room at once obviously).
No 15A circuits. Every circuit that is allowed to be 15A is also allowed to be 20A. I don't even own any #14 wire. (but I own 10 colors of #12, because I take color coding in conduit seriously, and since I work in EMT, green is not one of them.)