How can I remove Photo Library from iCloud without deleting the photos stored on the iPhone, so that photos will be included in an iCloud backup of the phone?
Yes you can!
Go to Settings > iCloud > Photos.
Make sure you have selected "Download and keep originals" turned on rather than "Optimize [device] storage". This way you'll have full resolution copies of your pictures on your device. You will have to wait for all pictures to download if "Optimize..." was on.
If there is more data to download than space available, you'll have to find a different route to keep your full resolution photos.
Or you may just turn it off and it will ask you to save the data if you had "Optimize..." on.
Then check Settings > General > Storage & iCloud Usage > Manage Storage and check that your pictures are being backed up.