This weekend, I was motivated to begin a series that builds upon this (and your other article Express.js on Cloud Functions for Firebase). I reference both in my article.
I spend a little more time on the “why use GraphQL” as I have been fairly skeptical myself. While, I actually have a fully implemented CRUD example backed by a Firebase database, I have only written the first article and have not gotten to it yet. Hope to wrap up the series in the coming week or so.