What about maybe adapting a 3-electrode type mV control that is often used for cooling tower make up water?
They normally have one common electrode, one to open the fill valve and one to close it off after the level is reached. You would need a good ground for this system to work and would also have to either cut off the electrodes at the height you needed or could try using the existing ones.
A contactor to control the pump motor would be needed since these aren’t really designed to handle motor loads. Baltimore Aircoil or Warwick Controls should still have these available.
