Hi … challenging issue indeed.
To confirm if I understood what you did for troubleshooting:
Do you mean that you replaced the complete hotend three times, i.e. tried three new complete hotends?
If that is the case, you can eliminate the thermistor.
Did you receive any HMS error code, either at the start or after following the self-test procedure?
In any case, you can measure the sensor resistance to confirm: HMS_0300-0200-0001-0002: The nozzle temperature is abnormal,the heater may be open circuit. | Bambu Lab Wiki
What is the daughterboard? Do you mean the interface or the TH board? If so, which one did you replace?