What you do is this
1) buy 2 external USB3 drives (a 2TB and a 4TB)
2) make the 4TB a time machine backup drive, call it backup
3) make the 2TB a media drive, call it media
4) make sure your iTunes preferences/advanced tick boxes are selected that say "keep media organized", quit iTunes
5) copy the entire folder "iTunes Media" from your home iTunes folder to the external "media" drive
6) start iTunes and go into iTunes preferences/advanced and tell it your new media folder location
7) quit and restart iTunes, test your songs, movies, ...that they play
8) now you can delete your old "iTunes Media" folder in your home iTunes folder. It is no longer used.
9) turn on time machine and have it backup both macintosh HD and media
IF you add music, it will go automatically to your external media drive. The artwork cache is still on the SSD so seeing your stuff will still be fast.