The following has information on turning off iCloud Photo Library. But be aware that if you turn it off and delete photos from the phone, you won't be able to turn it on again on the phone without the photos syncing right back to it. Get help with your iCloud Photo Library - Apple Support
My first choice (depending on the size of your library) would be to leave iCloud Photo Library on and turn on the Optimize Storage option if you don't already have that on. It will take some time and won't free up all of the space the photos are taking. It will try to keep as many full size photos as possible on the phone while allowing some free space as it is needed for other things.
If that isn't enough, your options would include turning iCloud photo Library off on the phone and then manually transferring future photos to your other devices or move completely away from iCloud Photo Library and use Google Photos. The following may help with that: True Photos and Videos Backup using Google Photos - Free and Unlimited
There are other options depending on what you want (what devices you have, how you want to transfer photos, etc).