Yeah the purpose of iCloud Photo Library is to allow you to have a large library all accessible from all your devices. If you have iCloud Photo Library on, and the setting to NOT store originals, you’ll achieve your original goal though. It will store only thumbnails on your iPhone, with originals in the cloud. I will LOOK like all your photos and videos are on your device, but they aren’t. It will intelligently use space to keep original copies of ones it thinks you’ll want to look at (like favorites). It will also free up space as needed as well. So you may see a large amount of space used by Photos, but as you need more space, it will intelligently free that space up on its own. In other words, turn on iCloud Photo Library, set it to not store originals on device, and it will free up space as you need more for other stuff. Then if you go to use a photo/video that’s in the cloud, you’ll see a low res version for a few seconds, until it downloads the full version from the cloud (and a little circular download status indicator).