I run a triple monitor display and one monitor for discord. I’m looking to add a small iPad or screen where I can run the PFD and maybe some other instruments on. What is a good software in order to achieve this?

Many, many years' ago, I used Splashtop Wired XDisplay. I have no idea if it would be suitable for you. https://www.splashtop.com/products/wiredxdisplay?srsltid=AfmBOoq-pqA_WStED9NUiwAizglU2mhYWCizccDmT4F8qkWiINU1mgT7