iCloud Photos on IPhone not syncing

I have pretty much the exact same issue you described.

(The only difference is I'm using an iPhone 16 Pro – not Max – and the same problem is happening on my iPad too).

Very frustrating, and multiple/recent software updates haven't fixed the issue. I've also tried disabling/re-enabling iCloud photos, which hasn't worked.

Sorry I can't offer a fix, but I'm hoping this boosts the signal on this issue.

It must be some setup or software peculiar to the iphone , which is why I suspected VPN. Some ad blockers have a VPN profile installed.

If you have all your data sync'd to iCloud (social media can be tricky) you could backup, reset the phone to factory and set it up without restoring the backup, then see if the photos sync. The thumbnails should sync maybe 10,000 images in an hour. If they do then it's software. Big job but may fill a boring weekend.

start here If your iCloud Photos aren’t syncing to your device – Apple Support (UK)

What VPN are you running?

PS turning icloud Photos off and on will simply take even longer.

Make sure iCloud Photos is set to optimize storage.

Even if you tap Resume Syncing on both uploading and receiving devices?

Not even overnight on wifi and on charge?

Stuck even after completely resetting the phone to factory? Then I am out of ideas. It could be a rogue image but that’s a needle in a haystack to find.