There is a collection of techniques that can reduce energy consumption of the Nano. (eg, remove power applied LED) I've read about µA's during sleep periods. The ESP-12F clearly achieves this and can be a MySensors by adding a radio. Note that if WiFi is not disabled, savings are probably lost because the WiFi is energy hungry. This would have me leaning towards MySensors rather than ESPHome. My experience tells me that the Nano is more durable than the ESP. Another tip: If you're going to put your sensor in a harsh environment, after you have assembled it, "paint" it with electrical lacquer.

@mfalkvidd After giving this some thought, wouldn't that imply that at sufficiently low flow rates, the the meter would not detect any flow. A leaky faucet? While I'm pretty sure that my pump can't pump that slow, the pressure would drop, eventually triggering the pump to start. A leak could be detected by periods of pumping that are very short? An experiment could be run to see how much water must be released to trigger the pump to start. The time between pumpings could then be used to determine this flow rate. And after reading this, I see I have to much time on my hands which could be used to address more pressing issues.

Stainless steel probes are a solid choice for durability. If you're thinking about power efficiency, integrating a sleep mode could really stretch out battery life.