One more update:
The Cox app doesn't appear to have device-by-device data usage anymore. Or at least I can't find it anywhere it's supposed to be according to Cox's website or other tutorials. It's not been helpful for troubleshooting.
The online data usage site shows my usage is about what it should be - roughly 12-20 GB/ work day, more on the weekends or if I download a big file. I still have no idea what caused massive data spikes seemingly at random, but it doesn't appear to be any of the devices on my network since they otherwise act normally.