After my disasterous update to El Capitan which tanked my iTunes Library, I am still working on cleaning it up!
I have fixed the music, and re-rated all my tracks, and only lost about 20 tracks out of my 40,000 plus itunes library, but now I am working on video and that is such a mess. So many tracks have been randomly renamed! I have to go through and watch some of them and figure out what they are and rename them!
All I can say is never move your iTunes library to an external Synology as it just ruined my iTunes library completely!