I have to concur with the other comments. I got a Luna USB-C key right before Apple announced sidecar and back then I was pretty bummed I spent the money on something I would not have any use for anymore once Catalina would be out. Turns out sidecar doesn't nearly work as well as Luna Display. Sidecar loses on all things that matter: image quality, user friendliness, responsiveness, stability. The only thing it wins on is price, hard to compete with free.
I recently found another use case for my Luna display by the way, when I wanted to play some real-money online poker in the garden (shelter in place makes you want to do things like that ;-). Apple does not allow gambling applications on iOS, and screen sharing Pokerstars on my Mac Mini using VNC was a really shitty experience. So what I do now is start the Luna app on the Mac Mini, connect the iPad to it, then take it out into the garden (which has its own wifi AP) and put it on full screen. Responsiveness and touch control are excellent that way.
I recently found another use case for my Luna display by the way, when I wanted to play some real-money online poker in the garden (shelter in place makes you want to do things like that ;-). Apple does not allow gambling applications on iOS, and screen sharing Pokerstars on my Mac Mini using VNC was a really shitty experience. So what I do now is start the Luna app on the Mac Mini, connect the iPad to it, then take it out into the garden (which has its own wifi AP) and put it on full screen. Responsiveness and touch control are excellent that way.