Using the Luna to let an iPad be the only display for a Mac Mini sounded so good that I went ahead and purchased the Luna Display USB-C dongle to use with a new 2018 12.9" iPad Pro and a new 2018 Mac Mini.
Just to set things up, I’ve plugged in an old widescreen HDMI monitor that was being used with a Win-7 machine. Plugging in the Luna I didn’t know whether the Retina option should be chosen in the Luna set-up on the Mini as the iPad has Retina but the HDMI doesn’t. I selected it but see no difference. Understandably, because, at least for now, the HDMI monitor is wide-screen, the image on the iPad is LetterBox format.
One annoyance is that each and every time I restart the Mini or after Luna on the iPad was brought to the foreground I have to go into the Mac Settings>Display>Arrange, and reset it to “Mirror” otherwise I just have a blank screen with the menu bar along the top. I wonder if there’s a way to make the “Mirror” option permanent.
Using the Apple Pencil 2 on the iPad with Luna to place or move the cursor works well. However using my finger on the iPad often doesn’t. Touching the tiny red circle in the top left to close a window often just doesn’t work, the Pencil-2 will save things; as will using the Mini’s Magic Trackpad 2 (and presumably a mouse on the Mini).
I’d say that if needing to use the Mac Mini when travelling you need the Mini, your iPad, your Mini’s keyboard, and either an Apple Pencil 2, a mouse, or your TrackPad, but NO monitor is required!
To date, I would recommend the Luna for those times a monitor for your Mac Mini is not available or to use your iPad as a second screen for your Mini. The Luna at $80.US is not cheap, but I don’t find it over priced for the job it does. I just wish the Setting>Display>Arrange-Mirror option didn’t reset to OFF all the time.