I've just come across a no-name non-contact, high-precision 80 GHz mmWave radar sensor for liquid level detection with a 40m ...

I was thinking there must be a simple sensor that could tell me when the water level has fallen a set amount and automatically run the "fill pool" zone on the Rachio.

Sure, modern paper shredders already have sensors that tell you when the bin is full, but if your machine lacks this feature, you can just build it a custom sensor with an Arduino Uno.