Photos app stuck on "Uploading 2 Items"

I'm having a persistent problem trying to sync the Photos app on my iPhone SE running iOS 14.2 with my iCloud Photos library. The library contains over 12K photos and several albums, none of which are shared, and all the photos themselves appear to sync successfully with the Photos app. However as soon as it gets toward the end of the sync, it gets stuck on "Uploading 2 Items" with a blue Pause link next to it and it has been this way for an hour now. None of my albums are showing up on the device. Going to icloud.com shows all photos and albums correctly so I'm satisfied that the photos are on iCloud. They're just not being completely downloaded to the iPhone properly for some reason.

I've tried: turning iCloud Photos off and back on from Settings, signing out and back in with my Apple ID and resetting the device in between and various combinations of these none of which work.

I've spoken to Apple Support and they think there may be one or more corrupted photos in the 12k photos but no idea how to identify them. Nor are they able to figure out what the 2 items it is stuck uploading are.

I don't want to take the nuclear option of backing up 12k photos, deleting the library and starting again but I will do it it's the only approach (and ideally there's an easy way to export all photos plus their metadata and edits), and reimport again.

Thanks,

Steven