Luna Display to use 27inch iMac Late 2014 as monitor for new M4 Mac Mini?

This is a screenshot of the Luna support page:

The red emphasis, is theirs, not mine.

To me that screams that it is not suitable for the primary monitor for any computer.

The US$80 cost of Luna will go a long way toward the purchase of a nice free-standing monitor.

The initial setup of your new Mac mini will require a dedicated monitor.

Sett up the Luna hardware/software will also require the mini to have a monitor.

You will save yourself headache and disappointment if you just buy a monitor to go with your new computer. They aren't very expensive, starting about $125 and going up from there.

doubt you will be able to have it as the main display and performance will be unlikely to be good for watching video and playing games

you can read more here

27" iMac as External Monitor for new Mac … - Apple Community