Astro HQ are the creators behind a small USB dongle, called Luna Display. Put it in a Mac and combine it with an iPad and it turns the iPad into a Mac display.
Pairing is very easy. You install the Luna Display app on both the iPad and the Mac. Then you start the iPad app. After a few seconds, you will see your Mac screen on the iPad. You can touch the screen to push buttons and manipulate things, but since the Mac was made to work with a mouse or trackpad and a keyboard, I recommend you use them as well. For my Mac mini, I use an Apple Magic Keyboard and a Magic Trackpad. The Mac itself is hidden behind a cover on a hallway wall. I can sit in the living room with the iPad, wireless keyboard and trackpad working in macOS.
My Mac mini has never had its own display, so Luna Display is a perfect solution to temporarily use an iPad when I need to use the Mac. My iPads are my 99 percent work and leisure devices, but I need to do things on the Mac mini now and then.
The connection between an iPad and the Mac feels instantaneous. I notice no lag at all (as with VNC tools). I probably could work fulltime with this type of setup.
A Luna Display dongle costs 80 USD. They come in two types, with USB-C contact, and with Mini DisplayPort. My Mac mini is a 2011 model so I use the Mini DisplayPort Luna Display.
If you want a cheaper, but not wireless, solution for using iPad as a Mac display, try the Astropad app (also by Astro HQ). You connect iPad and Mac with a regular Lightning to USB cable.
I almost forgot! Off course you can use the iPad as a second display if your Mac already has one. That was the primary use of it. 🙂
Do you have the need for a solution like this?