Create user in my database after creating it with Firebase

I have a Next.js frontend and an Express backend. The auth is handled by Firebase. Users are allowed to sign up with email/password or through Google.

I need to make sure that users also exist in my database, not just in Firebase, as I need a way to link the user to all other entities that exist only in my database.

Achieving this for email/password is rather simple, I just need an endpoint on my backend that creates both together, the firebase user through the admin sdk and a user in my database.

Handling this for the Google sign up is more problematic as I need to first create the user on the frontend. After calling signInWithPopup, I could call my backend with the uid of the newly created user and add it there, but what if my backend's down?

An option would be to have a middleware on the backend that creates the user if it doesn't already exist, but then I have to do this check on every request.

Thoughts on how to best handle this?

A workaround I can think of is using Firestore and Could Functions to keep retrying when your backend is down.

When your backend is down, write the user data to Firestore from your frontend. Then use a Firebase Cloud function to keep trying to write that data to your external dB. After succeeding, you can delete the Firestore document.

This is actually much simpler than expected. On the backend, I have an endpoint called /init that creates the user on the backend and adds to backend user it to the Firebase user's id token result. Then on the frontend, I have an auth guard which makes sure that the user gets initialized with the backend if he doesn't already have the backend id set in the id token result.