So after debugging for a few hours, I found the main issue.

There’s no need to change anything in your code. Just go to your Google Cloud Console, then remove your app’s OAuth 2.0 Client IDs and recreate them.

You don’t need to change the web application Client IDs.

Google has a limit on OAuth for production-level apps, so this will reset it—and voilà, you should be able to log in!