A few small findings just from my digging (I don't know if any of this means anything).

When I run a file verification on my game, it always finds one file missing. I searched Steam's log to find what was missing, and it says:

"0 chunks corrupt and 1044 chunks missing of 1064 total in file "Data\Patch0_Hotfix4.pak"

That file creation date in my folder is exactly when my game stopped working. I don't know if that's the issue or not, but it seems suspect, I think. My SO has BG 3 and theirs is working great. I may try to delete the file and let it redownload first. and if that doesn't work, I may copy their version of the file and put it in my game and see if that somehow fixes it.