I'm having the same problem and have been since I setup my phone 2 days ago. I've tried pretty much everything in this thread with no luck. The sharing option does seem to force the download of whatever you select, but this is tedious with a large library.
I also have all my photos backed up and synced with Google Photos. I've been hesitant to open the app as I wasn't sure what would happen since the photos weren't all on my phone yet. A friend of my mine suggested trying it, as they thought Google Photos would probably analyze each photo to compare it with my Google library and as it analyzed it, it would force a download from iCloud. I can't confirm this is what it's actually doing, but ever since I opened Google Photos about an hour ago, the number is actually moving. It's gone down by about a thousand in the last hour.
Your mileage may vary but it might be worth a shot, as Google Photos will attempt to at least touch every photo.