Have to be careful here. After making sure that all the photos are in iCloud Photo Library, you don’t delete them from the iPhone. Instead, you disable iCloud Photo Library and let the system remove them (it may take awhile depending on the number of photos and videos).
New photos can be added to iPL via your Mac either by manually syncing through Photos or Image Capture, (for photos) using PhotoStream or another 3rd party program like PhotoSync. Or you can re-enable iPL with “optimized” set. I’ve got 14K items (with s few weeks of full-size photos and videos) and I’m only using 1.78GB.
That said, I’ve seen some funky issues in the past where the system doesn’t remove some of the items when I disabled iPL. I tried manually deleting them but, when I re-enabled iPL, the deleted items were all deleted from iCL. I got them back from the Recently Deleted album but I had a few minutes of panic until I remembered they should be there.
I was just trying to force the iPhone to replace the current copies with optimized items to save space. I think doing this is actually to backup and restore your iPhone using an iCloud Backup.