55mn later, I re-imported the whole library in place so _at least_ everything in the FS is listed in beets' database.
Some albums are still split apart but that's fixable and I'll just fix it bit by bit rather than apply like 300 changes at once 😩
Turns out most albums were already migrated and it turns out I had some split/duplicate albums so I took care of that.
I just went the lazy route and deleted/re-imported duplicate stuff rather than try to make beets' `duplicates` plugin work. It means my "Recently Added" playlist is busted but honestly, I can live with that.
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