My thoughts are that dietpi-backup wants to change ownership of all files on NFS to root, which isn’t allowed unless you relax security on the Netgear ReadNAS Duo. This is because the only remote backup supported is via NFS.
It may be possible find a more restrictive security solution than just going with no_root_squash, but that still leaves NFS as the only remote solution. What about using additional shares like CIFS/Samba etc.? What about using additional tools like rclone to allow cloud backup to Backblaze B2 and Google Drive, etc.?
My challenges were trying to get legacy hardware and OS to make NFS work because of NFS being the only option, which would not have occurred if I could use other options.
I’ll update you if I find a better way to do NFS backups; can you consider other backup options also?