I'm running gamescope-session (the Steam client from SteamOS) on NixOS via Jovian. I tried both the 160MB and the 5MB versions, and tried both protontricks 35140 -q d3dx9 d3dcompiler_43 and protontricks 35140 -q calibri. If I click Play in Steam, the Steam logo appears for a second, and then I'm dropped back to the Asylum page in Steam.

I don't know how/which logs to collect to diagnose the precise issue.

I also can't get the original launcher to work after trying this either. 😰