If you are prepared to buy new ticket to CNX if you miss your FD flight to CNX (mind you, domestic flights in Thailand are fairly cheap even last minute, so not the end of the world) then by all means take the chance and go for the earlier flight - without luggage there is a fair chance that you will make it. There is nothing more annoying than booking the later Nok Air flight and then on the day of traveling realising that you could have made the earlier (and possibly cheaper) FD flight.