My iTunes Music Structue
Can someone tell me what I need to keep and what I can get rid of?
Mac mini (2018)
Hard to say, as we have no idea which folders or items your current library is pointing to.
You could potentially delete the previous libraries if you are certain you won't want them any more, but they usually don't take up much space as they are just the informational DB files without any associated media.
For anything else, you need verify what folders your library is currently using.
