Funnily enough, the crashes were Windows side, not Linux/Proton. Well, the stream was crashing on Linux, but not the game. But yeah, there are a lot of moving parts here, I agree.
Luckily, I got it working solid with Sunshine on Windows. Unfortunately, there’s not a Sunshine build for Tumbleweed, and I can’t be assed to compile it from source right now, lol.
Edit: Jesus, there are so many moving parts, I forgot which platform had which issue. I agree, there’s probably something funky going on with Proton and Remote Play. But I also got Sunshine working on Linux now, and it’s been flawless (after telling it which display to stream)!