Speaking of Gameboy on the Steam Deck, have you seen this effort of the GBOperator?
I feel like you’ll enjoy that link!
Sadly the price of Japanese consoles went up a heap over Covid, though you can still get some amazing deals with proxy bidding and buying services, if you take your time!
I haven’t ever been too picky with FPS when playing on the Steam Deck, so I’m not the best to ask. They have optimized it far more than the first KCD. And to me? It ran beautifully.
I had zero issues with it.
Here is the link to ProtonDB’s page for KCD II. Its a good way to see what a bunch of users have experienced, and the settings they’ve recommended!