iCloud Photo Sync

Why not simply have an option to "View the photos" that will be removed? It knows what the photos are, so just show them. And if the number of photos to be viewed is too many, a simple explanation of how to view them should be added.

What everyone wants to know is 'how do I know what photos are going to be deleted from the phone?'. They are still on the MAC but what are they.

You cannot do both, Enable iCloud Photos and Sync to a device.

Sync photos to your device

• Connect your device to your Mac. ... 
• In the Finder on your Mac, select the device in the Finder sidebar. ... 
• Click Photos in the button bar. ... 
• Select “Sync Photos to your device from” tickbox, then choose Photos, Pictures or a folder from the pop-up menu.
• Do one of the following: ... 
• Select syncing options:

More items...

Sync photos between your Mac and iPhone, iPad or iPod touch

Turn on iCloud Photos ; On your iPhone, iPad, or iPod touch. Go to Settings > [your name]. Tap iCloud. ; On your Mac. Choose Apple menu > ...

Set up and use iCloud Photos

And if the number of photos to be viewed is too many, a simple explanation of how to view them should be added.