Deleting photos from phone, but keeping them on iCloud

It sounds like you're just trying to free up space on your iPhone? If so, do not delete anything.

Navigate to Settings > Photos and toggle on Optimize iPhone Storage

This will remove the local storage of photos on your iPhone and they will stay in iCloud. You will still be able to view these photos on your iPhone as well. They will just have to load when you open them instead of them being readily available.

If you turn off iCloud Photos on your phone, I believe that all photos and videos that were in iCloud would remain in iCloud. You could then safely delete / permanently delete the local copies of photos and videos on the phone.

However, you say you would turn sync back on after deleting photos from the phone. That would cause the phone to download new copies of all of the photos and videos you had just deleted. There would be no space savings … and you might end up doubling the amount of space that photos and videos take on your phone, if the photos and videos that you deleted while iCloud Photos was disabled were still in the Recently Deleted album.

There is no way to use iCloud Photos synchronization on your iPhone while storing photos and videos only in iCloud. The closest you can get is to enable "Optimize iPhone Storage." This gives the phone permission to substitute low-quality local copies of photos and videos for full-resolution copies on an automatic, as-needed basis; the idea being that the phone can re-download full-resolution copies as needed from iCloud.

It is possible to offload some photos to a Mac – so that your iPhone and Mac synchronize some photos through iCloud Photos, but there are other photos that are stored on your Mac, only.

If you hold the Option key while launching Photos, you should get a dialog that lets you create or select a Photos library. When you have several Photos Libraries, at most one (the system library) is synchronized with iCloud. All the others are strictly local.

Apple doesn't make it easy to move photos from one library to another – you can't simply open two libraries at the same time, and drag-and-drop directly between them. You can do it indirectly. (E.g., select system library, export photos to Finder, quit, restart Photos to get into the local library, import photos from Finder.)

Once you had done this and backed up your Mac (to safeguard the photos in the local library), you could delete the copies synced to iCloud Photos.

If you were interested in offloading photos to your Mac (only), it wouldn't necessarily need to be to a second Photos Library. There are other photo organizers (e.g., Adobe Lightroom Classic), and some people even store their photos "loose" in the Finder. The basic idea is simply that if you copy photos to somewhere OUTSIDE of the iCloud Photos system, you can get those photos off your phone without sacrificing automatic synchronization of new ones.