As another user has stated, got this to work on Manjaro by:

1. copying the ~/local/share/Steam/steamapps/common/Baldurs Gate 3/bin/bg3.exe file to the Baldurs Gate 3/Launcher directory
2. renaming the LariLauncher.exe file LariLauncher.exe.OLD
3. renaming the just-coped bg3.exe to LariLauncher.exe.

After that, the game runs from Steam's native Proton.

Couple issues, primarily being that Windowed Fullscreen (called Fake Fullscreen in the menu) locks your cursor to the game even after trying to tab out.