What I want specifically isn’t just syncing whole libraries or whatever, but being able to have the same fine control that I could have if I plugged in my iphone, adding and deleting individual songs, etc.

I vaguely remember that being a thing, but not how to do it.

P.S. I have a lot of songs and itunes can be agonizingly slow at times, is there a fix short of conducting a great purge of my song library?