It sounds like most of your frustration can be eliminated by actually using your NAS as the ground-truth for all data, using something like SMB/NFS instead of SyncThing.
Then you only need to backup the NAS.
Then you only need to backup the NAS.
But if you’re on Linux, it’s better to use ZFS or Btrfs and native filesystem snapshots, since they’re atomic.
That gets synced and it's been trouble free so far.
git push nas