IPhone – Recovery/Restore of Photos

I'm hoping someone can help me with an issue with recovering photos on an iPhone. I'm an IT person (but mostly work in the Windows world), and am trying to help a friend. This friend had an iPhone 7 (probably 128 GB) and recently upgraded to an iPhone X. Unfortunately, it seems the person at the phone store screwed up helping them, and I'm thinking that they probably restored an older iCloud backup to the new phone. The reason I think this is because there's about a year's worth of photos that are missing on the new iPhone X. I happened to have a spare iPhone 8 64 GB temporarily laying around, so I did an iCloud restore of their most recent iPhone 7 backup to this phone and found all of their "missing" photos. Unfortunately, the problem now is that as the iCloud backup attempted to restore everything to this temporary iPhone 8, it ran out of space. I don't know enough about the workings of iCloud (restores, iCloud Photos, etc.) so am at a loss now as to the easiest way to transfer these "missing" photos to my friend's new iPhone X (especially in light of the fact that the temporary iPhone 8 is out of space and didn't finish downloading everything from the iCloud backup).

I was thinking maybe of trying to upload all the photos to Google Photos, or maybe letting it sync to a computer and then do something from there. I'm not sure, but it seems there is some sort of iCloud Photos repository in iCloud that allows photos/videos to be uploaded and stored in iCloud (my friend has a few hundred GB that he pays for) and then synchronized with multiple devices (similar to Google Photos), but I can't figure out how to get these "missing" photos to that easily (I'm thinking maybe they haven't synchronized to that yet because the iCloud backup restore never finished…?). Any help is greatly appreciated. Thanks!