I'm sure that would work, at least initially. I'm concerned about reliability. My water has very high levels of calcium carbonate which quickly scales up everything. I think the electrodes wouldn't last long. A nonintrusive solution would be great. I've seen nonintrusive water flow sensors that use ultrasound to detect the flow of water. They are industrial devices and their price makes them unsuitable for my use.
There are cheaper optical water level sensors in the aquarium trade that might work for you. Check out the Tunze Osmolator as an example. It could easily be connected to a GPIO on the pi.
That's a very interesting idea. Thanks for the suggestion. It is still invasive since I have to cut the intake pipe and insert a section with the sensor installed in it. But it is more flexible since I can install it in any size pipe. And for my use case of a 1 inch input pipe it can be cheaper than my current solution. For example https://www.aliexpress.com/item/15mA-5V-Optical-Infrared-Wat...
Although, I wonder if scaling (as in hard water scale) is still going to be an issue.
