Cull photos from cloud

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 devices/iCloud while maintaining a master copy on the computer. Added photos can be exported from the new Library and imported into the Master Library. Turn off iCloud syncing when reverting to the Master Library.

Photos - Use multiple libraries in Photos

Be aware of the following if you intend to continue using iCloud Photo Library. iCloud Photo Library is a syncing service. Any photo you delete from iCloud will also be deleted from all of the devices where iCloud Photo Library is turned on. See: iCloud Photo Library - Apple Support

Open www.icloud.com and select the unwanted photos , hold command key and tap them and click on download button .

if the external drive is connected , open finder select the photos that are in download folder ( to select photos in fractions hold shift key and tap them ) , then drag and drop photos to an external hard drive .