Unfortunately there is no way to guess the genre with mp3tag itself. You have to do with external scripts or the built in ones within mp3tag. But here are some examples from the 'Web Sources Scripts' part of the forum.
I have one for beatport.com one which is mostly for electronic music @ beatport.com scripts by stevehero (search & direct) - jaimz mod. I have attached screen-shots of what the script tags.
Others are:
Pone's iTunes script: /t/13478/1 Pone's Discogs script: [WS] Discogs (pone mod)Dano's Juno Records script: juno records
If you have mostly complete album then tagging them again should not be that much of a problem. All of these scripts get info such as 'Cover Art' 'Artist' 'Title' and your 'Genre' tag and many more. But you can disable all but tagging the genre tags with these scripts (I think) you have to see with each script. I know for definite that mine and Pone's Discog's scripts allow to customise the outputs.
Any more questions feel free to ask and welcome to the forum.