Well, Immich mentioned above, seems to do that.

I don’t know what you mean by “somewhere else”, but would not Backblaze as a backend for Nextcloud cover some of that backup scenario?

PS.
How do you handle it when the phone runs full?

If you delete after upload, you’d have to download it again if you need it 2 minutes later…

Great answer, thanks for sharing!

I’ll make sure to test on IOS before I put more time into the rest of the puzzle.

I’ll look into shared vs group folders :-)

Les Pas backup settings looks very interesting on first glance.
Setting up differentiated folders in Syncthing can be a bit fiddly.
But as you say, no IOS :-(