Delete photo from phone memory and place on external memory
I am using a Western Digital MyCloud NAS drive on my home network. The MyCloud app for iOS has the ability to sync all your photos from your iOS devices (iPhone or iPad, etc.) to your MyCloud. I have it set so that whenever my Phone is connected to a Wi-Fi network, it connects securely to home and uploads all my new photos. That way, I can take as many photos as I want without having to worry about it.
If you would rather, I see there are thumb drives now with lightning port connectors so you can backup your photos, videos, contacts and calendar info too.