Syncthing will do a 1 to 1 connection if possible, else it will use a relay server. Traffic is encrypted. Open source. After the initial setup of marrying the devices together, it's just a matter of starting the application. Pretty much what you want?
There are community-contributed relay servers that you can use. Check out the syncthing website, they explain how all of this works. It's a very good piece of software.
