Andrew, I am just curious if your OSPi config worked when you connected it to the sprinkler system? I have installed my OSPi in the existing Hunter X-Core enclosure to use its 24VAC transformer. OSPi board works fine judging by the green board light and measuring supplied voltage on VIN-GND and RPI pins. However, when I stick my preconfigured RPI 0 W into OSPi board and plug the power supply the RPI fries instantly. Burned two of them already. Created a ticket for support three days ago, but so far haven’t heard anything.
I have responded to your support ticket. Honestly I don’t know — if OSPi is outputting a voltage much higher than 5V, that could explain it, but you measured VIN-GND voltage, that’s about 5V, so it’s normal. I can’t think of any other reason why it would burn RPi.