Photos sync stuck

Is the library in iCloud complete? You may want to check this on your iCloud web page www.icloud.com in the Photos.app there .

If all photos are in iCloud and Photos is working well directly in iCloud, you could rebuild your library by downloading a fresh copy from iCloud. This will be faster then trying to open the current version, when it cannot get past "restoring".

Try this:

• Quit Photos if it is still running. You may need to use Force quit from the  menu.
• Create a new, empty library by launching the Photos.app while holding down the options key ⌥ . Select to create a new library.
• Now open the Photos > Preferences > General tab and click "use as System Photos Library".
• Make the new library your iCloud Photos Library in the Photos > Preferences > iCloud by enabling iCloud Photos.

Then wait for the photos streaming back from iCloud to your Mac. You may need to remove the old library to free the storage for the new library. But keep a copy of the old library on an external volume, just in case you will need it.

Rebuilding the library from iCloud has a few limitations. The projects will be missing, as they are not stored in iCloud. And some manually added faces may be missing, as Photos will have to scan the photos again for faces.

"Rebuilding the library from iCloud has a few limitations. The projects will be missing, as they are not stored in iCloud. And some manually added faces may be missing, as Photos will have to scan the photos again for faces."

I do not doubt the veracity of your suggestion. But have you considered the powerful and huge implications of what you've said? You've said, "Apple has created a critical software, used by millions, that is fundamentally flawed, is open to failure, and has no fix." That's tantamount to, "Yes it looks great. Sorry about the exploding gas tank and random acceleration." Thank God Apple does not build cars. ... Oh. Wait.