The Nextcloud iOS app does it. For some reason it requires the location permission "all the time" for that, presumably as a way to "wake" the app from time to time?

I decided to try Nextcloud exactly because of this. My problem with it is more that the whole thing is a bit unreliable. Like once in a while the app will get into a state where the only way I found to recover is to just erase everything and re-sync everything. And the app will resend ALL the pictures, even though they are already on the server.

And I can't do that with my family members' phones. It doesn't matter to me if the app takes a month to sync the photos, but it has to require zero maintenance. I can deal with the server side, but I need it to "just work" on the smartphones.