And how are you accessing it when away from home? A VPN that you're permanently connected to? Is there a good way to do NAT hole-punching?
Syncthing kind of does what I want, in that it lets all my computers sync the same files no matter what network they're on, but it insists on always copying all the files ("syncing") whereas I just want them stored on the NAS but accessible everywhere.
Nextcloud kind of does what I want but when I tried it before it struck me as flaky and unreliable, and seemed to do a load of stuff I don't want or need.