I have the exact same problem, same model trash can. Trying to zap the pram, it will do one chime but then shuts down. plus it’s running very slow. I erased the internal ssd and trying to install Big Sur. what the heck is going on?
I left the battery unplugged from the logic board for 24 hrs and charger disconnected. Pressed the power button for 10 seconds. Plugged the battery back in, pressed the power button, nothing. Then plugged in the charger, green light on the plug, pressed the battery charge indicator and all lights lit up. But still no start up when power button is pressed. Could the power button be broken? How do I test that.