I have a lot of pictures on my iPhone SE. I have them synced with my iCloud. However, every picture I add gets added to both my device and my iCloud. I don't want this. I don't have much storage so I want to delete all pictures I currently have from my iPhone and put them all on iCloud, so that I can start making pictures again! How can I do this?
You can't, is the short answer. iCloud Photo Library is a two-way street - if you want to be able to upload to it, it has to be turned on, which means photos will also be synced to the device.
You can enable Optimize Device Storage under Settings > Photos, but that will still use some storage on the device. There is no way to bring that to zero without disabling iCloud Photo Library on the device entirely.