Syncthing is great. It's peer to peer, so peers only sync if another peer is also online; some people 'solve' that by having an always on server as a peer, but it's not a requirement.
If you do have a server peer though, you can also use it to ensure that there's a second copy of all synchronised folders (other peers can safely have different subsets), and thus as the one place from which you can take snapshots to backup elsewhere. (Syncthing does have versioning a la Dropbox, but IMO just like Dropbox it's more about sharing between people/devices, and an automated regular snapshot should also be taken.)