How can I delete photos from iCloud without affecting my phone?

Unfortunately, you can't just delete photos from iCloud without it syncing to all your devices since iCloud is a syncing service. When you remove photos from iCloud, they'll also be removed from any device linked to that Apple account. You might want to consider turning off iCloud Photos on your devices if you're looking to separate them, but keep in mind that could lead to losing data if you're not careful.

Another thing you could do is manually back up the important photos you want to keep to your computer or an external drive, and then delete them from iCloud. Just remember, once you do that, you need to manage your files separately!

One option to consider is upgrading to a larger iCloud plan, like the 200 GB one, but I understand you might need permission from your family. Just make sure to check with them—it's often a good solution if you find you're frequently running out of space.