Quick Update on how I finally solved the problem:
I did all of the following but this time one after another WITHOUT REOPENING THE PROGRAM:
- Downgrading iTunes then reupgrading
- Deleting all iTunes files found in Finder > Music
- Resetting all settings and deleting all files (including cache)
Then, I copied an empty iTunes library onto a hard drive and held the option key down when launching Music. When it asked me to choose a library, I chose the empty one and everything worked fine immediately. I now have to launch the app with my harddrive connected, so it isn't a complete fix, but at least Apple Music works now. I will try launching through a local copy of the library in the future, but right now I don't want any more trouble.
Cheers,
Leon