Lower baud rate to 115200
While things appear to work OK at 230400 I do run into the odd issue from time to time that leads me to believe it's something with the serial connection.
According to this https://copperhilltech.com/blog/rs232-to-usb-converter-with-arduino-due-or-mega-2560/ the maximum baud rate for the Mega2560 board is 115200.
Interestingly this post https://arduino.stackexchange.com/a/6865 provides the following:
Baud Rate UBRR(dec) UBRR(hex) % of error
300 3332 0x0D04 0.0
600 1666 0x0682 0.0
1200 832 0x0340 0.0
2400 416 0x01A0 0.0
4800 207 0x00CF 0.2
9600 103 0x0067 0.2
14400 68 0x0044 0.6
19200 51 0x0033 0.2
28800 34 0x0022 0.8
38400 25 0x0019 0.2
57600 16 0x0010 2.1 - Barely OK
76800 12 0x000C 0.2 - GOOD
115200 8 0x0008 3.7 - BAD
230400 3 0x0003 7.8 - BAD
250000 3 0x0003 0.0
According to their calculations the fastest the Mega256 can go with a sub 1% error rate is 76800.
I think I'll have to do more digging.