Nevermind.
I finally found the solution, I'll detail it here if anyone else has the problem:
In my case, an old hotfix from patch 5 was in my data folder, accessible from the local files option in steam (Libarary>BG3>Properties>browse local files) Apparently it was not removed when uninstalling the game, so I missed it during my troubleshooting.
The only reason I was clued into this was because I tried to start the game from the launcher, which I normally have disabled in steam settings to save time. The launcher itself told me of the data mismatch, and this page:
https://larian.com/support/faqs/the-launcher-has-a-data-mismatch-error_78 told me where to look.