Store iTunes library on network drive, keep local cache of some media for offline use, and let iTunes organize media on network drive
I'm using Supersync on multiple macs that's based on my qnap nas that keeps track of everything -- from "cloud/not on this machine" "on thismsachine:notonserver" etc.

It also handles dupes, missing tracks, various library issues -- and even better fixes them easily.

I'm not shilling for them -- I have some problems with its quirks -- but with 4 machines, 15 years of iTunes and now a ton of other media that would dwarf my drive it does the trick.

Hope this helps...