iCloud Storage - No photos or videos but still shows as full
Go into the iCloud Photo Library submenu (from screenshot above) and select "Disable and Delete" to turn off iCloud Photo Library. Delete everything in your Photos app - don't forget to check shared photos and all albums. Then re-enable iCloud Photo Library.

iMessages also takes up space - clear out/delete old messages (the attachment images may be counted toward your photo allotment). May help, may not, but worth a shot.