I had this exact problem randomly a few months ago, and this fixed it.
The phrase was something like -skip launcher
This won't work here: even with --skip-launcher Steam still runs the launcher, it just doesn't appear to the user.
The problem we face with hotfix 11 is that a dll is missing in our Proton prefixes (CefSharp.Core.Runtime v117.2.40.0) and the launcher immediately crashes.
I'm not sure about the degree of control one has on the Steamdeck, but on a regular Linux desktop one can just symlink bin/bg3_dx11.exe to Launcher/LariLauncher.exe to actually bypass the launcher.
