Deleting photos from iPhone and keeping them backed up on iCloud

1. A common misunderstanding: The iCloud Photo Library is not a back up. It's a sharing service.
2. Once you turn syncing back on, the iCloud Phot Library will - as the name suggests - sync with your phone and the images deleted from your phone will also be deleted from the iCloud Photo library
3. Many people overcome this issue by using Google Photos in addition to Photos.

As Yer_Man says, iCloud is a synchronization service. It's purpose is to share pictures with other devices, not to be a backup. When you engage iCloud Photos the Library on that device is kept exactly the same as the iCloud Photos Library. If you delete a picture on your Mac, then that picture is deleted at iCloud and on all the other devices. So you can't think of iCloud Photos as a full backup service, since it backs up mistakes.

So, if you turn off iCloud, delete pictures, and then turn iCloud back on, Photos will see the difference and make them the same by re-copying the deleted pictures back to the iPhone. This is different from what Yer_Man said, but I think I'm right. I just tested it, and that's what happened--I duplicated some pictures to delete them, but afterward the duplicates came back. But I was turning on iCloud pretty quickly after deleting-- timing may count.

But, in either case, it won't do what ou want. I have not experience with Google Photos, but I know you should be careful. Some people have gotten all twisted up trying to synchronize two services at once.