Sounds like NFS might still be the way to go for you.

For backups personally I use Restic and connect over SFTP via SSH, since that’s just built in and doesn’t need any configuration.

For more traditional file sharing I use WebDAV with SFTPGo, since I need windows and android compatibility too, and webdav is pretty easy to setup and use.

And I also use Syncthing for keeping some directories in sync between devices.