How Can I Delete Photos from iCloud Without Removing Them from My Devices?

Unfortunately, iCloud is meant to be a syncing service, not just a storage dump. If you delete from iCloud, it deletes everywhere. Your best bet would be to disable iCloud Photos on your devices to keep the photos stored locally while clearing out your iCloud.

If you disable iCloud Photos on your devices, you can delete what’s on iCloud without affecting your local photos. Just keep in mind this will also stop new photos from syncing to iCloud.

iCloud Photos is designed to sync your images across all your devices under the same Apple account. If you really want to keep photos on your devices, you might consider turning off iCloud Photos on those devices first. You can then download any full-resolution images you need, but just be aware that this will make your iCloud backups larger since you'll be storing originals there instead of just optimized versions.

I understand where you're coming from. If your iCloud is full, it might be worth moving less important photos to your device while keeping more significant items in the cloud. Nevertheless, this setup defeats the purpose of having iCloud as a backup service.