If your main issue is protecting the pump, perhaps investing in one with its own dedicated line-voltage float switch is the way to go. In other words, separate the problems and make your life easier. Protecting the pump doesn't need to be part of level monitoring. Instead, pump protection should be a proven, self-contained and 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 level monitoring will be installed from the top.
That in mind, here's some ideas for monitoring level:
- Multiple optical prism sensors, in a dip tube. Simple, inexpensive, no moving parts, could give direct drive to LEDs or some simple display.
- Capacitive tank sensor, used in the RV and boating trades for fuel, water and wastewater monitoring. Again, for your cistern, in a dip tube. No moving parts, but more expensive than optical sensors.
- Measure distance to water with a time-of-flight laser. Seems problematic. Could maybe be hacked to use a reflective float (use a cat's eye reflector?) Probably expensive.
- Hydrostatic (pressure) sensor, suspended from the top. Popular. This is arguably the best method for cisterns. Lots of off-the-shelf choices from modest to expensive.
With the hydrostatic approach you have lots of options for display, including wireless monitoring. With a vertical tank the math is simple: pressure is proportional to tank fill height, so you can calculate % of full, or give capacity in any units you choose (gallons, cu. ft, liters, m^3, etc.) With suitable math processing you can calculate dynamic usage, and even detect leaks.
This applies to capacitive and laser sensor approaches too.
More here: Is it possible to do water detection with a single electrode?