When starting a Proton game steamtinkerlaunch automatically generated a Kill-Switch script (under /dev/shm/steamtinkerlaunch/KillSwitch) which will stop the started Proton game immediately by stopping its wineserver. This works very fine and closes all game processes reliably.

The script can be called from everywhere (f.e. I have it on my Stream Deck), but is also available as Kill Proton Game menu entry in the Tray-Icon