Hey, I worked on this project for ~2 years, though I'm no longer with Shopify.

We started with Doorkeeper and gradually switched to building our own OAuth2/OIDC implementation over time, partially using glued together lower-level libraries like https://github.com/nov/openid_connect

Edit: I forgot, I even have a few small commits to that last project from my time at Shopify: https://github.com/nov/openid_connect/commits?author=meagar