How to delete photos from iPhone but remain on iCloud

Sorry, you don't. iCloud is a syncing service to sync data across all your Apple devices. If you remove pictures from one device, it deletes them from all devices including the cloud storage.

As a convenience to enable syncing content across your devices. And, to obtain space if you perform iCloud backups.

Yes.

However, if you want to retrieve those photos and you perform a restore, it will wipe out anything current, including new photos and revert back to the point in time when you made the backup.