The Admin API can be used to export user data. You won’t have the passwords for security reasons, but will be able to get the rest of the user data.
https://www.mongodb.com/docs/atlas/app-services/admin/api/v3/#tag/users
While not explicitly about getting user data, you can follow this tutorial to get started with the Admin API and Postman.
As far as a replacement, Auth0 is a strong option for authentication, and they have a free tier. You can refer to their docs for more information about how to import user data.