Reopen Photos with the option/alt key held down and create a new system Library on the external hard drive. If necessary, open Photos, go to Photos/Preferences/iCloud and set up syncing. Then go to System Preferences/iCloud, sign in, and check Photos syncing. Allow plenty of time for the photos to sync to the external drive. Once all the Photos have synced to your external hard drive, 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 select the Library on your internal hard drive as the system Library. You can now delete photos from your devices/computer/iCloud while maintaining a master copy on the external hard drive. Added photos can be exported from the internal hard drive Library and imported into the Master Library. Turn off iCloud syncing when reverting to the Master Library.