Sallymary

Go to System Preferences > iCloud > Options and uncheck mark "Desktop & Documents".

Add your Desktop and Documents files to iCloud Drive - Apple Support

Then click on Photos > Options and uncheck iCloud Photo Library.

Get help with your iCloud Photo Library - Apple Support

Open Photos, go to Photos/Preferences/iCloud and set up syncing. Then go to System Preferences/iCloud, sign in, and check Photos syncing. Once all the Photos have synced to your computer, quit Photos. You should now have a complete compilation of your photos (like a Master Library). Reopen Photos with the option/alt key held down and create a new system Library. You can now delete photos from your computer while maintaining a master copy on the computer. Once you have deleted all the photos from the new Library and iCloud, follow macjack's advice to turn syncing off. You can then delete the new, empty Library. Reopen Photos with the option/alt key held down and select the Master Library as the System Library.