That makes sense. My suggestion would be to not use an Arduino at all. You’ve already got a Pi in the system. From your description it sounds like the Pi can do all of the functions that the Arduino would do – in addition to controlling the OS. If you eliminate the Arduino, you’ll have a simpler hardware (and likely software as well) setup. I’d eliminate the Arduino and then write the code to monitor the pressure and take the desired actions all from the Pi itself. I’m thinking the code would be independent of the OS code. If your pressure isn’t where you want it to be, you use the API to turn off the running programs and maybe you also need the relay to shut off the pump. The relay can be controlled by the Pi.
Mike