I just finished reading through the entire thread because I was curious about the use of the conductivity of water as a level sensor.
Last summer I planned to upgrade my garden irrigation system to use a similar system. It is fed by our domestic water well. I cannot afford to overflow the tanks and create a lake. Has happened before.
So, after buying two of the conductivity sensor boards on Ebay, I built a sensor probe similar to what the OP has, but used float switches. One at the bottom of the pvc pipe to indicate the tank was nearly empty, and one near the top to indicate the tank was almost full. The top of the pipe is fixed to the top of the tank.
Using a Nano to watch the two switches and control a SSR that controls a solenoid valve to fill the tanks. Worked flawlessly all last season.
My suggestion to the OP is to change his sensor stack to use float switches to indicate the water levels. Then the off/on is truly solid on or off.
Paul