Yeah I guess my issue is that if I can't trust the mobile app not to lose my photos (or stop syncing, or not sync everything), then I just can't use it at all. There is no point in having Nextcloud AND iCloud just because I don't trust Nextcloud :D.

Nextcloud mobile app is crap but fortunately it’s just WebDAV so you can use any other WebDAV app for synchronization.

That's a good point! Are there good WebDAV apps synchronising, say the Photo gallery on iOS, transparently and always in the background?

Unfortunately Apple puts extremely strict restrictions on background tasks so you will never have something as seamless as native iCloud or the amazing Android FolderSync app that I used for realtime synchronization for several years without a single issue.

I know people work around these iOS limitations by setting up springboard widgets that piggyback on background refresh tasks to do uploads. People also create Automator actions (e.g. run every day at time or location based) in the Shortcuts app.

I haven’t tried it but a popular option on iOS seems to be: https://apps.apple.com/app/photosync-transfer-photos/id41585...