Have you heard of [syncthing](https://syncthing.net/) ? It syncs a folder between devices, like Dropbox, without the need for someone else's computer. No cloud, it's just your pc talking to your phone, directly and automatically, through your home WiFi.
I mean, Syncthing is much more than that. The great thing about it is that it works no matter where you are - home wifi, over the internet etc.
But that means that someone else’s server is used whenever you leave your home network.