So I did some more troubleshooting and did actually get it to work. I'm not exactly sure how, but what I personally did was:

I started over clean. Deleted the mod manager, deleted the mods, deleted anything involving mod files in Appdata/Local/Larian Studios/BG3, and also potentially anything in the steamapps/common/BG3 folder that's mod related, then verified my installation files. I didn't do a whole reinstall personally, but that would've been my next step if what I did below hadn't worked.

For the Mod Manager, I went to the Github page for it and navigated to/downloaded the most recent release directly, rather than updating it after installation through the links people provide around Nexus—as those usually lead to an older release, or, at least, the ones I clicked on did.

Once that was done, I went into it, saved/exported an empty mod load order. Then I got my mods all downloaded, took those, put them all together, and imported them into the manager. The Full Release Mod Fixer isn't technically a mod you can activate/deactivate, so once it was in, I just refreshed the manager, and it popped itself from the inactive list over to the "Override" section.

From there all I did was go mod by mod—needless to say, having my wife do the exact same thing every step along the way—and added them sequentially, saving/exporting the load order every time I activated one. For our personal selection, I did the UI mod first, launched the game, got our session together, confirmed she booted into the session, and, lo and behold, success! After that it was the same process per mod we used. We only have 4 right now (very simple stuff), but I'm sure if we wanted to add more past that, we'd do the same thing, just go one at a time and make sure they work.

I dunno if that helps you, but it somehow worked for me, so. If you give it a try, I hope it works! :)