I don’t have an exact answer for you but I can 100% confirm it works through Heroic.

Switch to desktop, install heroic launcher, install BG3 with default options to internal SSD, launch with Proton Experimental, it will say you need .net so accept the dialogue and download the exe, use Heroic to install the .net exe to the game’s prefix, go to the configuration again and add --skip-launcher (two hyphens at the beginning) to the launch commands.

That’s it, I did that and ran the game from desktop and then used heroic to add it to Steam. Now I launch the game straight through Game Mode. I also added Heroic as a non-steam game so I can launch it occasionally for patches.

Skipping the launcher is important, as is installing the correct .net exe. I’ve read that some folks had the game claim to be installed but then realized it never actually completed successfully because they ran out of disk space during the final step. It’s a very large install. I also read a one-off comment that power tools can break it, if you have that installed through decky loader.