So a little note here is that both the extruder and the heated build platform are powered via the extruder controller. If the extruder works, then we know we have 12 volt power.
Power is current times voltage= watts. You have 12 volts and the variable is current. Current is determined by resistance of the circuit path. The MOSFETS used on the extruder controller are very good devices with extremely low resistance, which is why they never get hot. 99.999% of the time, MOSFETS work or they don't. This isn't that 0.0001% error because it does turn off and on.
That only leaves wiring and connections. The power has to go somewhere and thus, whatever part of the path that has resistance is going to get hot.
First place is to check the screw connections at the extruder controller where the red, black, and blue wires go to at the extruder controller. Make sure they are tight and the insulation is not melting (sure sign of a bad connection). Next is the connector at the heated PCB. These are known to get hot as they are not the best connector for the job and will turn yellow from the heat on the pins. The heat makes the cheap tin coating oxidize thus the resistance goes up and even more heat happens in a self feedback loop. Catch it early and you might be able to clean the pins with steel wool before it gets out of control.
You said you replaced the heated PCB so as one more option, maybe the new ones have more resistance which means less current and less power or heat. Because they were burning up the connectors from so much power, maybe MBI changed the boards? This means there is nothing you can do without some major mods to the electrical system. The only way to get more power out of a given resistance is increase the voltage (Ohm's Law 101). The only way to do that is another power supply and a relay to isolate the extruder controller.
