[2021] Launcher through Steam in Ubuntu works, game opens for split second then crashes.

If you run the CK3 executable directly from a terminal command line, what output do you get when it fails then?

Please attach here your Documents/Paradox Interactive/CK3/pdx_settings.txt .
From your Documents/Paradox Interactive/CK3/logs/ folder, attach here system.log , error.log

Also in Steam please click on Steam-Help-System Info, and paste everything there into a text file and attach that here.