came back curious to see if the discussion took a different direction from besides sarcasm or another 30 posts saying “why not syncthing?” - glad to see the couple to comments including OP and yours as constructive.
that being said, i’m a syncthing user, including running my own (st) discovery server on openwrt. aside from some annoyance at rather frequent conflicts and being browser based, im running it on all 5 major OSs, including ios (mobius sync) and android.
i strongly disagree that running it on a phone is a pain, and in fact, found it the most reliable and versatile sync solution for ios by far - and that includes icloud, dropbox, google drive and google photos.
the only thing that comes close is apple photos, but that’s specifically for photos. and that too, only because of the deep os integration, ie always running in the background and allowing seamless access to older photos that are not on device. even then, there’s always a mysterious slight difference in # of items reported on the mac vs the phone - “eventual consistency”, where “eventual” is t=infinity i suppose.