So it sounds like you are saying Luna Display is worth the money because sidecar does not work with Mac-to-Mac. Right?
So, I live, everyday with Luna display. It is one of those products that is truly amazing. Let me give you my background on this device.
I have a work Macbook, which is nice but I want to be able to use a bigger monitor. I also have a 27" iMac, which has a brilliant 5k display. I don't have space for another monitor, although I was offered one from work. Enter Luna Display. With the luna dongle in the work Macbook, a lightning cable (although you can wifi across devices as well), and my iMac, I now have a 5K display attached to my work Macbook. Yes, you read that right, Luna's latest update powers full 5K at 60hz. The refresh rate is slightly lagging, but I hardly notice as I'm mostly jumping between Office365 apps.
I thought of getting another monitor, but as long as Apple continues to support the intel iMac, it's hard to leave the gorgeous 5K retina display.
Sidecar is, um, janky at best.
I bought the Luna dongle several years ago. I suppose the sale on it is over by now, but I think it's the best $100 I've spent on my setup.