iCloud vs photos on phone

iCloud only sync's content across devices. So any changes you make on any of your devices, including deletions, is sync'd with iCloud and all other devices.

You can offload pictures to your computer storage or an external drive first, then delete them, to free up space in iCloud.

No read here Set up and use iCloud Photos - Apple Support

For what you want to do, you need a different cloud based solution such as Dropbox, OneDrive, or Flicker.