Firebase cloud functions : Delete multiple documents on userDelete

I would like to be able to delete all of a user’s documents when deleting his account. I would like to do this on the backend side with cloud functions.
This similar post doesn’t really answer my question (because I need to delete documents from multiple collections)

Here is the structure of my collections:

users 
  | uid
      | email: [email protected]


orders 
  | docId
      | createdBy: uid
      | …
      | …

Here is my function:

const userDeleted = functions.auth.user().onDelete(user => {
  // get user doc in users collection then remove it
  const usersRef = db.collection("users").doc(user.uid);
  usersRef.delete();
  
  // get all user’s doc in orders collection
  const ordersRef = db.collection("orders")
    .where("createdBy", "==", user.uid)
    .get()
    .then(snapshot => {
      snapshot.forEach(doc => {
        ordersRef.doc(doc.id).delete();
      });
    })
    .catch(err => {
      console.log("Error getting documents", err);
    });

  return Promise.all(usersRef, ordersRef);
})

I need to iterate through the orders collection and find the documents that have the correct uid in the “createdBy” property and then delete them. I feel like I can’t delete multiple things at once during onDelete, and I couldn’t find anything concrete in the documentation.

What am I doing wrong?