+1 for Syncthing. I've been running it for years, after my student discount for Dropbox expired (Google drive and OneDrive were just getting traction at the time).
The mobile experience last I tried was pretty rough though. I don't really need my files on my phone and I have a web interface on my home server I can use to grab them in a pinch, but it's something to keep in mind.
You are most likely in France: your government does not allow publishing an app containing cryptography (in this case, Golang's crypto implementations and a package used by Syncthing - only using iOS libs should be fine) without authorization (which can only be obtained through French forms, at which point I'd want a French lawyer to be involved, so no).
You could of course build the app yourself from source.
The mobile experience last I tried was pretty rough though. I don't really need my files on my phone and I have a web interface on my home server I can use to grab them in a pinch, but it's something to keep in mind.