Whitelisting IPs of a Node.js App (hosted on GAE) to access MongoDB Atlas (hosted on GCP)

I want my backend server (Node.js, hosted on Google App Engine, flexible environment if that matters) to be able to grab and pass data from my database (MongoDB, through Atlas, also hosted on Google Cloud platform, in the same region as my backend server). Ideally, I'd like to keep a tight whitelist of IPs that can access my database, but I'm not sure how to identify them.

My understanding is that Google will use a range of IPs. I might be able to access these by querying Google from time to time (Google App Engine - list of IP addresses?).

I also found a nice tutorial from Google about how to connect App Engine to MongoDB Atlas, but they conveniently left out how to whitelist the correct IPs (https://cloud.google.com/community/tutorials/mongodb-atlas-appengineflex-nodejs-app).

I also found some notes from Atlas about using network peering within GCP (which I qualify for? see link: https://docs.atlas.mongodb.com/security-vpc-peering/) but there are significant restrictions, including that other IPs aren't allowed? I'm having a hard time understanding their documentation.

Key Question: Is there an easier way to do this than the first link above? Or am I stuck querying this frequently and changing it by hand?