I didn't think syncthing supported encrypted replicas - that's why I passed on it years ago for a dropbox-like use case. I wanted to have a replica in the cloud as a relay, without having to fully trust the machine. A quick google suggests they may have something beta along these lines, so this may change.

I believe Resilio can do it, but it is closed source.

If you're looking for backups and not dropbox-like functionality, then something like restic might be more appropriate. It does actual content addressed backups, so you've got history for those times when you realized you messed something up six backups ago.