How to delete my photos from iPhone and save space without deleting it from iCloud...?!

My iPhone has run out of space And is deleting all the apps and uninstalling everything. I can’t free up anymore space with deleting messages etc but I can’t free up the crazy amount of space taken by all my a old videos and photos because when I delete the 16,000 photos/videos from my iPhone it will permanently delete them from my iCloud and all devices. This just doesn’t make any sense to me as I thought the point of iCloud is to backup and free space up on your mobile devices. Can anyone help me as this has been going on for 6 months now with me deleting every tiny thing I can every week so I can use my phone at all.