One-sided iCloud photo upload
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.