I followed the example of the other person on Facebook and did a factory reset. Luckily I have most of my stuff on the SD card but i'm still going to have to set up a bunch of stuff again.
But more to the point, it worked!
I did a fresh install of BG onto deck storage (not SD) just to be sure (which still took forever cause 122GB) but I'm in and playing!
Found something else yesterday after rabbitholing down the steam forums. Now you solved your problem already. But hopefully that keeps it from coming back.
Currently there seems the a problem with PowerTools installed and the Game.
So if the game is crashing after the Nvidia logo (on experimental or hotfix) try to remove PowerTools and restart the Steamdeck.
If you tried a bunch of other proton versions, you may need to clear the compatdata dir for the game and then run again with experimental. The directory can get in a weird state with some games if you try to run multiple proton versions - i.e. one applies a fix on startup that breaks it in another version.