Firestore replica to another database

We are developing an app infrastructure that relies on Firebase services. Since we need a database and the data model it's not much complex, using Firestore should be the logical choice.

That said, Firestore is still in beta and we fear that once our app has grown, Firestore could no more suit our needs. In order to prevent this scenario, and only for writing operations, we think to not call directly Firestore from the app, but rather to transfer the request to a cloud function. The cloud function itself will write to Firestore and to another database (ie Cloud SQL or similar) in an atomic transaction to ensure a replica of Firestore Data.

This approach, we think, could have two benefits: protect yourself from a scenario in which Firestore no more fit our needs and to feed a web application which we will build after the app to host dashboards, reports and aggregations.

My question is: - is this approach a good solution? - there are other tools to pipe data to Firestore and another DB maintaining the operation atomic?