One of the issues with importing releases into MusicBrainz is identifying those releases that you have that are not currently in MusicBrainz in the first place.
I added a task to SongKong called Add to MusicBrainz that trawls through your music collection looking for folders that have albums not matched to MusicBrainz, but with good enough data to add to MusicBrainz and then adds a MusicBrainz Add to Release tab for each album found
This is free with SongKong Lite, no license required. But the idea is you match your songs to MusicBrainz first using either SongKong or another MusicBrainz enabled tagger like Picard, then run Add to MusicBrainz afterwards to add the albums that could not be found in MusicBrainz. SongKong also matches albums to Discogs, so this also provides an indirect way to import Discogs releases into MusicBrainz.