iCloud is a synchronization service. It makes sure that the Photos Libraries on your Mac and on your other devices are exactly the same as at iCloud Photos at iCloud.com. So if you connect your Mac Library to iCloud, all of its pictures will be copied to iCloud, and all the pictures at iCloud Photos will be copied to your Mac--except for obvious duplicates.
If you want to may iCloud Photos empty, then you need to make an empty Library on the Mac, sync that with iCloud, and then delete all the pictures on the Mac. That will cause all the pictures at iCloud Photos to be deleted, as well.
So
- Make a new empty Library -- close Photos, option-click on the Photos icon, and Create and name a new one.
- Make the new Library the System Library in Photos' Settings>General. The System Library is the only one that can connect to iCloud, so your other Library will be safe as long as it's not the System Library.
- Connect the new empty Library to iCloud in Photos' Settings>iCloud. You can set it to Optimize Storage, since you don't care about these pictures, anyway.
- Wait. Maybe days. Until Photos is fully populated from iCloud.
- Select all and command delete!
- You probably want to select all in Recently Deleted and erase them, there, too.
- Wait until you're sure iCloud Photos is empty.
- Change your Big Library to be the System Library in Photos' Settings>General.
- Connect the Big Library to iCloud in Photos' Settings>iCloud. You should NOT set it to Optimize Storage. Instead set Download Originals so that you will have the full set available for backing up.
- Wait some more.
What do you think?
