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.