If your main issue is protecting the pump, perhaps investing in one with its own float (if the pump is an immersion type) or flow (if it’s external) switch is the way to go. That’s the only thing that needs to work at line voltage.

In other words, separate the problems and make your life easier.
Protecting the pump doesn't need to be part of level monitoring. It should be a proven, low-tech approach. 

With that out of the way, I'm going to assume you don't wish to cut any additional holes in the side of your cistern; that any monitoring will be from the top.

With that in mind, here's some ideas for monitoring level:

 * [Optical prism sensors][1], in a dip tube
 * [Capacitive tank sensor][2], used in the RV and boating trades, again in a dip tube
 * [Hydrostatic][3] (pressure) sensor, suspended from the top. This is arguably the best method: no moving parts, gives continuous level

With the hydrostatic approach you have lots of options for display, including wireless monitoring.

More here: https://electronics.stackexchange.com/questions/700725/is-it-possible-to-do-water-detection-with-a-single-electrode/700742#700742


  [1]: https://www.electroschematics.com/optical-liquid-level-sensor/
  [2]: https://tankedge.com/downloads/How_our_exterior_Moda_tank_sensors_work.pdf
  [3]: https://blog.wellaware.us/blog/how-to-monitor-water-level-in-a-tank-complete-guide