Restic is the winner. It talks directly to many backends, is a static binary (so you can drop the executable in operating systems which don’t allow package installation like a NAS OS) and has a clean CLI.
Kopia is a bit newer and less tested.
All three have a lot of commands to work with repositories. Each one of them is much better than closed source
proprietary backup software that I have dealt with, like Synology hyperbackup nonsense.
If you want a better solution, the next level is ZFS.
You can consider something like syncthing to get the important files onto your NAS, and then use ZFS snapshots and replication via syncoid/sanoid to do the actual backing up.
Or install ZFS also on end devices, and do ZFS replication to NAS, which is what I do. I have ZFS on my laptop, snapshot data every 30 minutes, and replicate them. Those snapshots are very useful, as sometimes I accidentally delete data.
With ZFS, all file system is replicated. The backup will be consistent, which is not the case with file level backup. With latter, you have to also worry about lock files, permissions, etc. The restore will be more natural and quick with ZFS.
I can't speak to zfs but I don't find btrfs snapshots to be a viable replacement for borgbackup. To your filesystem consistency point I snapshot, back the snapshot up with borg, and then delete the snapshot. I never run borg against a writable subvolume.
All three have a lot of commands to work with repositories. Each one of them is much better than closed source proprietary backup software that I have dealt with, like Synology hyperbackup nonsense.
If you want a better solution, the next level is ZFS.