I'm not much of a gardener but the idea of measuring the moisture level seems ill-conceived... Namely because you are measuring at only a single point in your 3D region of soil, and you don't know what range of moisture should be maintained without experimentation.

Using our guy's own sawtooth model of watering/drainage, it would make more sense to just water at fixed intervals and experiment with the frequency to see if the plant grows.

Still a fun project!