Probably a personal character fault. You're right that it is overkill. And I'll probably be tossing some more money towards SparkFun soon too - but the just from being a pure-software guy to buying/assembling the raw parts just shut me down :-(
Welcome to engineering. Pick one and finish the project. Then look back.
Yes, running an RPi Zero to watch a button press is overkill. So what? It optimizes delivery time over efficiency. You can optimize for cost or power later if you feel like it.
Anyhow, nice job.
At some point you'll want to use the Pi Zero for something else, and at that point you'll be able to use some other IoT package.