If you don’t mind going full back-to-basics, you can do this with standard command-line tools and no cloud server. Contrary to popular wisdom, a server is not necessary to sync files between a computer and a mobile device.

I use ssh and unison over wifi hotspot, no cable required. Works fine though it does require a button to be pressed. It doesn’t sync constantly in the background. Personally, after many years of doing that, I decided that it was an anti-feature.