Genres are a big pain in the butt! I have spent many many hours tagging my library in ways that make sense to me, but I still do not like many of the choices. It would certainly be nice to be able to just download the genre tags from the web for everything (and you can), but I find them all stupid and confusing. Just TOO MANY DIFFERENT GENRES USED that do not make any sense.
MusicBrainz uses things like "Art Rock" and "Classic Pop And Rock" that to me are just stupid. My car uses Gracenote, and those fools actually show some individual songs on an album with totally different genres than all the OTHER songs on the same album (when there is truly no difference in the style of the music). And to make matters much worse, if you ever want to play your music with something other than MB, like a phone app, DLNA server or car system, most apps and music players do not deal well at all with MULTIPLE genres assigned to the same track, and that makes it important to chose the right FIRST genre in the tag if you ever want to find your music again.
So if you cannot find a site that just uses only the genres that you like, then your only real option is to manually change the genres and genre groupings in MB and then start tagging all the songs the way you want. MB is very flexible in this regard, but I still do not know if something from Alannah Myles is Vocal, Adult Contemporary, Hard Rock, Pop, or "Aor" (another MusicBrainz wonder genre), or maybe even "Aor Classic Rock". I would love it if someone could point me to a better solution to identify the BEST correct genres!
I have looked up complete lists of genres on various internet sites, and they are simply mind-boggling. Even with a list, someone still has to decide which ones to really use for consistency.