For iPhone and iPad: MobiusSync (which is a compatible, but non-free implemenation). Works great, just as SyncThing does.
edit: seems to support live photos and exif metadata… I have a feeling some formats or metadata might be lost though, but not entirely sure…
* iphone -> icloud
* icloud -> mac
* mac -> syncthing to the backup server
Standard backup policies from that point on.
I just found out about this tool from this (recent) thread: https://news.ycombinator.com/item?id=27758045