BasicMicro

So yes, multi-unit is enabled
If 10k is the minimum, then 12k is also fine then? Why lower resistance for higher Baud rates?

With the single Roboclaw, and the ESP32, I was still having issues with not reading the return signal until I used the pull up resistor and logic level converter. I haven't had a chance to fully experiment just using pullup resistors yet, but I was thinking/assuming that the logic level converter was also acting as some sort of signal booster/stabaliser?
Anyway, it seems to be an issue particular to the ESP32/RoboClaw combination

I have it working functionally if not optimally now, so can work on improving

I was also considering multiplexing the serial from the ESP32 to see if that helps instead

No. Higher resistor values mean less pull-up. By minimum 10k is the weakest pullup I would recommend. Maximum pull-up would be 1k. 12k is weaker than 10k. I would not recommend using 10k on baudrates higher than 38400. 12k should work at 38400 but I recommend 4.7k for 115200 or even 1k.

Depending on the length of wire a lower resistor value may be needed for a particular baudrate as well.