From [dargzero], https://www.protondb.com/app/397540
Before each launch of the game delete the following directory, otherwise it won’t launch.
<steamapps_dir>/compatdata/397540/pfx/drive_c/users/steamuser/My\ Documents/My\ Games/Borderlands\ 3/Saved/Config/WindowsNoEditor
The game tends to freeze up periodically. The following Steam launch options worked for me to reduce the number of times it crashes:
PROTON_NO_ESYNC=1 %command% -NoStartupMovies -notexturestreaming