I have to assume the constant failures due to breaking wires are caused by the fact that the thermistor wires are not properly supported and joined onto quite thick cables.
The most simple fix is to do the above hole of 2mm and to put a replacement thermistor with heatsink plaster (not the stuff for your CPU!) in there.
In the later steps, after fixing the firmware problem, I will explain how to use more common and cheaper alternatives by adjusting the tables for the thermistor.