Stored energy goes up. That energy comes from the work done to pull the plates apart (since there is a static electric force attracting them to each other)

If you hold the charge constant and capacitance goes down, voltage will go up (U = QC). The energy is (E = U^2C/2), so yes the energy will increase.

That extra energy is coming from your mechanic effort of separating two plates that attract each other.

Ok wow that makes sense. So I’d actually have a hard time pulling them apart?

What’s the limit? What happens 1 meter apart? 20 meters?

As long the capacitors have a width much larger than the distance between them, the energy stored is (if I recall correctly) linear. this is because the electric field is a constant vector field between the plates.

Once that is not true, you will transition to just separating two charged objects. Use Coulombs law to figure out the force. (Work energy is integral of force over distance).

The practical limit is not having electrons flying off due to the photoelectric effect or discharging due to conductive material in the environment.

So do it in a vacuum, in complete darkness.

Much shorter distances can be useful: https://en.wikipedia.org/wiki/Microphone#Condenser_microphon...