Let go, Let Google...Photos
Yes, while iCloud is convenient for sharing your photo library between iDevices and making sure that you have things automatically backed up. It's a bit of a challenge to delete a photo from your phone, without it also being deleted from iCloud.
Well the solution is easy -- Google Photos automatically syncs with your iDevice Library: uploading pictures to the cloud, so that you can delete the original and free up some more space on your iPhone or iPad. Google Photos isn't the only one that does this, if you are an Amazon Prime user: Prime Photos is an alternative solution. And there is always setting up your Dropbox to auto sync with your iPhone Library as well.
Needless to say -- get the originals off your Phone. There is little to no time we are left without Wifi or LTE that we won't be able to pull them from a cloud based storage solution.
Photo Storage Tips for iPhone
Here are some other useful iDevice Photo tips to make sure you are optimizing your storage!