For those who selfhost their music services, what are your must have plugins for beets and/or Musicbrainz

This is just a small list, but plugins like beetcamp, deezer, and lastgenre among others have been helpful to tag music. If you use deemix or buy music from bandcamp those plugins are extremely helpful. Lastgenre needs some configuring and monitoring to make sure irrelevant tags aren't assigned to songs though.

If you're into scripting, calliope is another tool that can interact with Spotify, lastfm, and musicbrainz (though musicbrainz stuff in my experience was unreliable). I'm using calliope to scrap together a solution to sync my local favorites in my music library and sync that to Spotify. It can also create playlists for Spotify from your local music library, and get Lastfm recommendations based on an artist or song.