Your current approach sounds fine from the perspective of bitrot. The hole in your approach right now is protecting from incorrect updates made either by yourself or by an application to your data that you don't notice immediately.

For example, say you are using a financial application like GnuCash on your desktop and it has bug in the software that gets triggered causing a bad write to the file. (power goes out, OOM, whatever..) syncthing will happily propagate that changed file that contains the bad write to your server and when you copy to the 3rd disk you will also propagate that changed file. You deleted what was on the third disk, so now you no longer have a "good copy" of the file.

If you add some sort of snapshotting into your backup routine, then you would be protected from this, because even though you would still propagate the change to the most recent backups, you could still go back to an older snapshot (maybe a week or a month ago or whatever) and pull back a working version of the file.