- With Nextcloud you have the option to encrypt the data at rest Nextcloud - Encryption.
- 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.