Totally get not wanting to swap the board if it’s that recent and been in serviced for that long. Super strange it’s seemed to have drifted that much, could think of some last ditch things like changing the position of the thermistor, ensuring it’s centred in the hole, set screw tightened enough but not enough to deform the sleeve but I know you’ve rebuilt and changed all the components a bunch of times, honestly could even just be your batch of thermistors, but it sounds like you have a way forward now.