Forward note: I am running Pop!_OS Linux (an Ubuntu derivative)
When I start up LightBurn, it comes up with ttyS0 by default each time, and I have to much around to make sure that I get it set up each time – and there are 40 different ports typically set up by the server…
I poked around and discovered that while I cannot set a default that will consistently work, I can however set up an exclusion list named PortExclude.txt in the LightBurn configuration directory, and add all the ones I know it cannot be. Fortunately for me, when a new USB device is plugged into the system it creates devices by the name ttyACM*. So, I added all the names ttyS[0…39] to the PortExclude.txt and now the only devices I see are the new ttyACM ones. I know that on other Linux distributions the default name is ttyUSB, but this solved the problem of having LightBurn selecting from a very small number of possibilities.
Hope this helps…