Using steamtinkerlaunch installed through the aur, I forked a custom command that launches EpicOnlineServices.exe to workaround error 200_19002 in fall guys. I also inserted a wait time of 3 seconds.

Using this and GE-Proton7-42, the game works fine. You can also edit the UnityPlayer.dll with a hex editor to change the aspect ratio to 16:10. Search for "39 8E E3 3F" and replace it with "CD CC CC 3F"

You also need to check if the fall_guys_client_game.exe is in the fall_guys_client.ini

Needs work to be more easily installed.