Syncthing is just open source Dropbox, self hosted.

It's entire purpose is "make two folders identical".

It's very good at that: so good that I frequently wish it did other things - i.e. if it had some notion of minimum seeding levels so it would destage files off a device provided they were replicated elsewhere (e.g. automatically clearing old photos off your phone would be a good use of it).

Yeah, I think I was just trying to get it to do something it wasn't suited for!