I am a little puzzled that you have problems with genres and subgenres after I participated in this thread:
Also, I think that a discussion about the discogs data quality would be a thing for the discogs forums. MP3tag is here only the messenger.
If you are not satisfied with the retrieved data and you want to adapt it to your needs, there are a number of action types that will help you to replace strings and split fields. I doubt that a ready-made function will be implemented for that as I bet that there are just as many users who want to keep the data as it is.