As you have learned, iCloud is a synch service and not a data archiving service where. The purpose of iCloud is to synch data across all your devices. Having said that, you can free up some space on your MacBook Air by choosing optimize photos.
Open Photos app ➔ Preferences ➔ iCloud tab ➔ Put a checkmark in OPTIMIZE PHOTOS.
This should free up most of the 50GB storage space that photos is taking up on your MacBook AIr. Once you activate this function then high and original resolution photos will be stored on iCloud and low resolution photo representatives will be stored on your MacBook Air. This will generate space on the MacBook Air. When you display a photo on your MacBook Air the the high resolution equivalent will be downloaded from iCloud and displayed on the screen.
Axel F.