I ended up using an ESP32 Feather from Adafruit along with one of their RTC feathers for better time keeping. I tossed in a 5V regulator and a barrel jack splitter in order to provide power to all of the boards out of one jack. After that I modified the case for some basic input and testing, and rigged up the GIANT alarm horn to an NPN transistor in case I ever wanted to use it (it did a short stint as a sound bomb, lovingly dubbed the 'annoyatron')