you will have no luck with any sensor of AC, because you have no port placed significant below water level in the normal pressurised reservoir.
You also can't go with the Koolance water level sensors which work with a reed contact, because of their length.
You could maybe fit the infrared sensor, but you will miss any way to use and display the signal with any AC product aka Aqauero and so on.

So no luck there.

in the sceanrio where the waterlevel will be low enough that the pump will run with a lot of air bubbles, you have two main actions which will appear:

1. The flow will decrease dramatically
2. The pressure right behind the second pump will decrease dramatically

Both actions you can measure and control with Sensors AC will offer you.

Also you will have an audible difference caused by the air. But even you missed that, your pump will not run completely dry*, so no worries there until the alarm action of an aquaero kicks in.

*Don't qoute me on that. You should not run the pump in this state for a longer time than necessary to shut down the computer. Without draining the pump by dismantle, you will have enough water for emergency lubrication**
**Don't qoute me on that either