I’ve updated my iOS, restarted devices and Wi-Fi and my phone will still not screen mirror to my iPad

iOS for iPhone and iPadOS for iPad are not AirPlay receivers - and therefore do not support screen mirroring between iOS/iPadOS devices. As such, you'll not be able to use your iPad as an external display for your iPad - or vice versa.

Consider that your iPad is generally able to run the exactly the same Apps as your iPhone - and can therefore be used to directly access and display the same content as your iPhone - and display this material on a much bigger screen.

The iPad is not designed to "receive" Airplay content. Like the iPhone, it can only send to a TV or similar device.

~Lyssa