Take a look at Syncthing. I've been using it for a couple of years for on- and off-site replication of similar-sized datasets. It's been pretty stable for a while now, with binaries for Windows, Mac, Linux, Android and so on. Has some interesting versioning features that might assist with your use case too. No VPN required. You can use a public introduction server or run your own. Not particularly difficult to set up.
Personally I use it to synchronise between the contents of done Veracrypt containers and several different computers at different locations. A reasonably good private "Dropbox".
Open source, fairly secure, active dev community. Well worth checking out.