Thank you very much @vexch, I think only with the help of your last question on the bug tracker it became much clearer to me why it always was rather disappointing to try out existing NAS distributions.
I hope my answer can also make it much easier to others here to make sense of and understand the motivation of this thread than my prior technical details:
[ From the bug tracker (edited) ]
Do you know any NAS software that uses the /home folder as a share or uses an UPG based private/group folder structure?
Unfortunately, I don’t know any readily configured NAS software, yet, that provides a user /home share that allows for simple group collaboration out of the box. FBX might be the first.
But I’ve seen such configurations, e.g. at schools (NFS or AFS? with corresponding SMB access), that is why it feels so disappointing to see how most NAS in the wild only seem to be used with a global public share and maybe one or two others without facilitating general user and group collaboration, because it’s too much of an administration hassle to set up groups and all needed combinations of shares and permissions.
[There shouldn’t be any unsolvable problem when sharing the server’s /home.] As long as the NAS drive isn’t actually mounted as /home on the clients, I think the server’s /home behaves just like a separate directrory tree on the client (while, where desired, it also allows the network-mounted-home-dir use-case).
BTW to add perspective: Maybe the idea of samba’s [homes] feature (https://www.howtogeek.com/howto/ubuntu/share-ubuntu-home-directories-using-samba/) could be described as “admin helper with user isolation” (the sharing space still needs to be administered separately), whereas a NAS /home fileserver that can be used as separate network sharing filesystem or as central /home server, may be the “complete admin simplification (made obsolete) that includes simplified user collaboration (resurrected debian UPG scheme)”. And the simplification would also cover the client side, as there is only one share/url/mount/network drive etc. that needs to be accessed or set up on clients.
So I hope the freedombox developers can appreciate the benefits of combining my successful experiments (to set up debian compatible user-private-groups on a freedombox) with a good default /home share configuration: Freedombox to become the first really-complete NAS solution, that supports direct individual user- and group-collaboration that starts on the universal filesystem level without any admin or user hassles (out-of-the-box).