have you tried syncthing? i have it running on a synology nas that acts as the always-on server part, but i used syncthing just fine for over a year without it as well. you just have to be very conscious about having both devices turned on at the same time so they can sync to each other. you pretty much have to do the same thing with dropbox anyway, i.e. let a large file upload to their server before you turn off your computer