LOL the “horribly wrong” is the part that worries me. We already have good mp3 tags for most of the collection, so I don’t want to make things worse. Thanks for the info, though.

Most commonly the issue is that it guesses the wrong album release, and puts any extra tracks in a separate compilation album. The songs are still tagged right, but the album is wrong. The worst problem I've encountered is when all songs were tagged completely wrong (different names etc.). Happened once or twice for me, but enough to not want to do everything in one go.

It seems jellyfin is confused by songs that have two artists, like duets. It can handle it at the song level, but those “merged” artists appear (as a separate artist) at the artist level too.

I don't have this issue. I separate the artists with a semicolon, so it is displayed "Artist 1, Artist 2" with each artist being clickable to go into their individual artist page. But I think you could actually tag 'artist' as "Artist 1 & Artist 2" and 'artists' as "Artist 1; Artist 2", and it will show up correctly, i.e. displayed as "Artist 1 & Artist 2", but shown in the artist overview separately as "Artist 1" and "Artist 2". I think...