Cool, you fixed my issues!

The audio output still doesn’t default to the deck speakers, but after following your hint it is now at least easy to select it while in gaming mode.

I installed the decky loader, and as non-steam game Audiotube. It seems to work, doesn’t matter very much to me if keyboard shortcuts like next track don’t function, I usually listen to full albums or dj mixes of 1 or 2 hours.