How do I offload my photos from iCloud to free up storage?

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 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.

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

Yes, you can import them to your computer and move them to an external drive for safekeeping:

Import photos and videos from your iPhone, iPad, or iPod touch - Apple Support