Ok… So I had a bit of a quick hack just now and managed to get it working. I connected the 5v pin on the DXL MKR Shield to the VIN (5v) of the teensy4.1 and it worked.
I also deleted the line where I begin the serial1 port and the code worked the same as with - I don’t think it’s needed.
Serial1.begin(TTL_BAUD);
I have only tested the LED on and off function so far but I hope this helps you too.