I need help with my 2018 i7 Mi Air 13.3

It no longer boots. When I connect the charger, even if I don't press the power button, it beeps once and the power LED on the power button lights up as does the keyboard. The fans turn on, but nothing happens on the screen. No backlight, no icon... nothing.

Same thing happens when no charger is connected and I press the on button.

In both cases, it "stays on", with the LED glowing for hours, if I don't press the power button for 10 seconds.

I have another, working Notebook of the same model, so I tried swapping displays, SSDs and even the Mainboards. The problem stays with the Mainboard.

I'm at my wit's end. Is seems to be stuck somehow before even loading BIOS.

Any suggestions what to look for?  Voltages at all measuring points around CPU and GPU match the ones from the working one.