That was mostly my point, yes. If you consider there to be a "Steam garden" on the Deck, then Windows is a much worse one - when even Windows is fairly open already.
There is no unofficial jank, not any more than there is any "official" non-jank on Windows. You use third-party programs to access third-party games and software in either case. And everything I heard so far about Lutris was good. Heck, the unofficial Linux EGS client has more features than the official Windows one.
About the only thing you'll really gain from using Windows is Gamepass. And you'll get all the overhead from Windows background processes and services.
As for the latter questions - I expect it'll work with relatively few issues, the integrated gamepad probably identifies as a regular gamepad in the system and it's just down to AMD to provide the proper drivers, but... about the only game store you'll be able to comfortably use will be the one that actually bothers with UX for gamepads and touch - i.e. Steam.