I'll happily recommend https://syncthing.net/, it's open source, end to end encrypted, and peer to peer (your machines send files directly to each other).
>I remain unconvinced of the usefulness of the Syncthing sync model on iOS
https://forum.syncthing.net/t/syncthing-for-ios/16045/4
So basically it's not an alternative at all to Dropbox
And to be honest, most of the files I sync, I never access on a phone anyway. So it would still be useful even without Android support.
Not sure how common my setup is, But I have desktop both at home and at work. And I use git and synching, to sync most of the stuff seamlessly. That said I mostly work from home nowadays, so I mostly use my work comp to speed up compilation, or run tests, dockers, databases, etc.
* Not trying to be an asshole. There are plenty of iPhone only app, that I sometimes** wish I could use, so I know how you feel.
** But not too much, otherwise I would switch by now :))
(I don't have proper backups beyond Dropbox, because I'm pretty happy for stuff I've actually deleted to stay deleted. Just, in case of a hard drive failure or something similar, I can sync back from Dropbox. That's all I feel I need. Maybe there's something better?)
With end-to-end encryption you can transfer data with an untrusted server as intermediary, which syncthing doesn't support.