That J5 connector is the MCU reset line, if you short it out it will simply reset the ESP32 which will stop the steppers, drop the serial connection and kill the PWM output. It will also lose the position in the running program, lose the position of the axis’, lose the inputted operation file and de-power the stepper drivers allowing the Z to drop under gravity.
There are no spare pins on this board that would allow you to configure a feed hold function which is what you really need to pause the job without losing its position and preventing the Z from dropping.(there is another thread discussing just this on this forum here
The same info applies to FluidNC, you could re-purpose the Z limit switch pin to be a feed hold pin by changing Z:motor0:
limit_neg_pin: gpio.34
to
limit_neg_pin: NO_PIN
and
safety_door_pin: NO_PIN
to
safety_door_pin: gpio.34