Well said, and the 'oops shit's dead' is also in the making.
We have the chance to be close to a school so electricity + internet is never down. The time to move to a VPS/other location is minimal, with encrypted backup on multiple locations.
Open to suggestions to improve the setup.
it's the same as hosting your Google Drive, you juste have to create different user and it won't be shared between user, except if you share them...
for backups, I would suggest you look at eather Plugins or do your own rsync. If you go serious about this, donc forget the 3-2-1 Backup Strategy
To have access as network drive, theirs always the possibility to install it as a Desktop app. Or you could mount a network drive with Nextcloud and use it as an 'interface' to see the drive.
As for a guide... Google, YouTube is your friend...
Theirs those if you want to start
My personal advice, if you setup as a docker, use an external database and not the one provided within the docker, it might break when you'll update Nextcloud.
I never tried it but many this one? https://docs.wedding-share.org/