Using iPad as second screen without sidecar I have a MacBook Air 2017 which is not compatible with sidecar, and since I upgraded to Catalina it doesn't work with the third party service I had been using (DeutDisplay). 

Does anyone know a way to use my iPad Air 2 as a second monitor for my computer?
Astropad and their luna display hardware and app are what you want when sidecar isn’t fast enough or doesn’t support your hardware.


https://blog.astropad.com/astropad-vs-apples-sidecar/
https://lunadisplay.com/


I’ve used Luna since the kickstarter days and you should be able to find one used if you need that specifically instead of the full AstroPad app. The new hardware is excellent as well, so give them a chance if you like what you see.

I used duet before and the astropad performance and experience on the Mac side were substantially faster, more reliable and used less cpu / energy.