I have a 200amp service meter wired to a 200 amp outdoor breaker panel which has lugs feeding my 200amp breaker box in the house. I'm adding a garage 30 feet away.
Can I put another set of lugs in the outdoor breaker panel and run 200amp to the garage, or do I have to downsize the amps and if so what is the largest amps I can run?
Since this is a garage you're feeding and not a house, you can't use the "83% rule" (aka Table 310.12 in the current Code), as that rerate only applies to feeders or services serving entire dwelling units. As a result of that, and the fact that a 2/0 copper wire is rated for 175A (a standard breaker size) at 75°C, you're stuck upsizing this feeder to 3/0 copper hots and a 1/0 neutral, or chasing down a 175A circuit breaker for your panel if you can't enlarge the feeder wires.
The easiest way to do this is to use a subfeed lug kit to tap the busses on your panel. For your panel (Square D HomeLine), the correct kit is a HOML2225; this includes both the subfeed lug block itself and a neutral chair lug that adapts your fat 1/0 neutral to the panel's busses.
The bad news is that you have a Square D HomeLine panel. While normally a reasonable choice, your complaints about salt air and aluminum are a problem for this panel, as it has busses made from tinned aluminum. If you want a copper bus option, the Siemens PNW0816B1200TC is reasonably priced but'd require you to get a different breaker and lug block, while the Square D QO1816M200FTRB is somewhat more expensive, and also requires different parts, but might be easier to mate to the existing conduits.
Yes, that's fine. If the panel manufacturer supports double-lugs for the output lugs you could use those. Otherwise, use 3-port Polaris splices to fork the wires. Note this all needs to happen inside the enclosure, and the enclosure needs to be big enough.
You would need 4/0 Al wire for the feeder. (Normally you would need 250 kcmil, the next size up; however your service is 200A and serves a residence, so a downsize is allowed.)
I realize this doesn't directly answer your question, but I'd consider swapping your meter base to a class 320 service. That way you can run 2 200 amp panels at full capacity. Class 320 meter bases aren't cheap, but since you said you weren't worried about cost, that would give you the most robust solution. You'll need to involve the power company to make sure the feed and transformer is up to feeding a class 320 service. The POCO is always the long pole in the tent!
Since the new garage is only 30' away, wire cost wouldn't be a major consideration. You'll have 2 "main panels", one in the house, the other in the new garage. Not only that, you wouldn't have to mess much with the existing setup because all the new stuff will be "before" your outside breaker panel.
Heat pump, EV charging, arc welder....hmmmm, those aren't light loads.
So in summary, my suggestion is, even if not fully responsive to your question is: Swap the meter base (If supported by the POCO), to a class 320, run your 4/0 and 2/0 to the garage (in conduit of course) to the new 200 amp panel in the garage and you're done.
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