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.
What the author describes here is hard because it's "simple."
What's simple because it's "hard" is replacing parts 2 & 3 with a network appliance like TrueNAS running a zfs pool that syncs to backblaze every night. Yeah you have to learn a bit but it won't fall in weird ways like the hard drive part here will just fail to mount one night and not back things up for 3 months until you notice. My 2¢