[EDIT FIXED]: apparently musicbrainz_albumid is actually mb_albumid in beets, but I would never have solved this without your lead in the right direction. Thank you!
Since that seemed so easy; honestly speaking I should have asked about using this tag instead. It will work 100% of the time whereas that one only works if it has a catalog number. As I tag, my files get populated with this tag:
I’d love to use it in the same command, but throwing in album.musicbrainz_albumid didn’t seem to take