If you're on linux you can use id3tool

I did this a while ago for incorrect tags on my Plex and it worked well.

https://manpages.ubuntu.com/manpages/lunar/en/man1/id3tool.1.html

If I understand correctly you would just need to put all of the files you want into one folder, navigate to the folder in your command line and then run id3tool -G Jazz *.mp3.

It operates off a list of predetermined genres which you can identify by running id3tool -l

Furthermore the *.mp3 will apply it to only mp3 files, if you have other formats and would like to capture all files simply remove make it just * instead.

Sorry I can't be more specific as I can't fully remember, but I thought I'd comment in case it helps.

For downloading a bunch of music, especially more obscure stuff, soulseek might be worth a try - if you search a genre you like you can then explore the libraries of the posters - some are very extensive. Just make sure you follow whatever guidelines they set out on their profile before you download a bunch of stuff, definitely make sure you're sharing some music yourself on the platform and that your listening port is set up otherwise you might get blocked.