@parsec, what I’m about to describe may be the consequence of bringing Windows install prefs.ini and prefs.ini.old into a Mac but regardless, here goes …
When I first load Lightburn after replacing the prefs files I get this screen with the Start button.
The first time I choose a listed device and then click either OK or Unlink Library I get the following screen.
At this point the Start button disappears.
Any subsequent time I choose the Device
I simply get the Laser window sans Start.