By now I've learned to use mp3tag in conjunction with Discogs. I joined that community and have learned one thing fast: if they don't have my particular release, I can add it, so long as I follow their guidelines (which are eminently reasonable).
The problem: mp3tag cannot retrieve titles or artists from subtracks. Whenever a release has a piece of music in multiple movements--or a single "opus" has multiple pieces associated with it (say, Chopin's Preludes, Opus 28, Numbers 1-24), Discog's Guidelines specify that the tracklist show an index track, containing the overall name (say, "Preludes, Op. 28" or "Symphony No. 6 in B Minor ("Pathetique")") and a subtrack for each piece that comes under that name (say, Etude No. 1, 2, ... or 1st, 2nd, 3rd, and 4th Movements). The mp3tag program will extract tags from regular standalone tracks only. It will not retrieve tags from index tracks or subtracks.
Lacking any better solution, I have learned how to build spreadsheets in advance, showing anticipated filenames, artist names, album and track titles, genres, etc., and then export these to CSV (comma-separated-value) format for use with mp3tag's Convert routine for text files. I've done this twice already, and I believe I'm close enough to mastering the technique. Which is very helpful when tagging 131 tracks from a 12-disc boxed set! (Or re-tagging a 12-disc CD box.)
But I would like to suggest that maybe mp3tag's tag-extraction routines get an enhancement that lets it pull tags from index tracks and subtracks.
A Discogs index track and subtrack set will always have the beginning of the track title as the index track title. Each subtrack would complete the title with its own title field. Thus, if the index track held the title "Preludes, Op. 28," and a given subtrack held the title "No. 7 in A Major," then the title would be a concatenation of (index track title) + ", " (a comma and a space) + (subtrack title). The resultant from this example would read "Preludes, Op. 28, No. 7 in A Major." The subtrack would hold the artist, unless the release has a collection of artists at the bottom of the track list, with track positions for each. Even then, mp3tag's batch-processing feature lets the use make short work of tagging several files at once for a given artist.
This is only a suggestion, nothing more. I know how to build text files for this sort of work. But in case anyone can't do that so easily...!