Perfect timing, I literally just got an RG350P in the mail 5 minutes and this is one of the first things I want to try!
Just downloaded a bunch of games and it seems to be working perfectly fine!
As you said, some sound effects don't seem to work correctly yet (as I noticed in celeste), but that probably has nothing to do with the 350P specifically.
Wonderful work on this, I am so happy to finally have a good physical pico 8 device!