while ( Serial.available() ) Serial.read();
Seatalk NG is actually same as NMEA2000. There is a simple adapter cable to convert Seatalk NG to NMEA2000, but as far as I know it is just cable with different connectors - so no logic on it. Do not mix Seatalk and Seatalk NG. From Seatalk to Seatalk NG you need converter or device, which supports both like I have Autopilot. Then you see at least some Seatalk data also on Seatalk NG and NMEA 2000.Waht is mqtt broker? If you do not yet have any Arduino board, I prefer Teensy 3.2 or up.