The hot bed circuit is pretty isolated from the logic circuits. Because of this, I don’t think that the board was faulty and hooking up the heated bet did it.
So, places where 12V can get to logic circuits:
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At the hotend, +12V goes to the heat block and the heat block is near the hotend thermistor. A short in the thermistor insulation can short +12V to the logic circuits.
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At the heated bed. Ditto. It might be a bit more difficult, but if you have a PCB heated bed, the thermistor leats might scratch the insulation.
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Endstops. Endstop pins are directly connected to CPU pins, and are definitely NOT high voltage tolerant. Heck some of these things can smoke a CPU with 5V. So something like a bad endstop configured to short power to the signal pin could do bad things indeed.
These would be my first choices to check.
Methodology: Disconnect all of the connectors from the mainboard, but not from the printer.
Use a spare Dupont pin to connect to one lead of a multimeter. Check all of the logic level leads for continuity to the heated bed and the hotend heater block. Anything where resistance is not infinite is a problem. In your case, because the problem became apparent when you turned on the heated bed, that is probably the prime suspect for where. I would most particularly check for continuity between the leads of the heated bed and the heated bed thermistor.
At least there’s a good chance that the TMC2209 drivers survived.