Three part answer.
1. 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/computer 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
2. One option would be to open Photos with the option/alt key held down and create a new system Library on the external. Once everything is downloaded, reopen Photos with the option/alt key held down, and switch back to the Library on your internal drive to delete photos. Switch as required/desired.
3. One option to free up space would be to move your Photos Library to an external hard drive. Then quit/reopen Photos with the option/alt key held down and make the external hard drive file the system Library. Once you are sure all your photos are there, you can delete the Library from the internal drive if desired. Once you do this, the external hard drive will need to be attached to access/sync your photos.
Article if for iPhoto, but the basics are the same.
iPhoto: Move your iPhoto library to a new location
Photos library - Move your Library to an external storage device