I ended up with the solution using an ESP32 but I hacked a dev board. What you learn is, the RS232 device chews up lots of power, so much so you don't want to keep it onboard. I used a single cell LIPO battery and a low-dropout regulator to make the 3.3V. It's been discussed above that a simple switch consumes the least power, which is what I used. The ESP32 has certain PINS that can wake it up from a deep sleep, so use one of those on your switch. Basically opening the door wakes up the processor, then I sent a text message to my phone saying "you've got mail".