How to use multi Auth - firebase?

Question

I have a register screen that contains "username, email, phone number, Password" and in this case, I use Phone Number Authentication to verify the number so after user verify his number I save his data into firebase DB,

so after that, I navigate hem to login screen! that should contain Email, Password "he registered by them before"

So I don't just compare if his data exist in DB or not, So it should be used Email/Password Firebase Auth,

But I think it's will take a lot of hits to my Bill or something,

so what you think to achieve these cases because I'm forced to register by Email For Reset Password later?

here is my register Code

 signUp = async () => {
    const {phoneNumber} = this.state;
    this.setState({message: 'code was sent'});
    const phoneWithAreaCode = phoneNumber.replace(/^0+/, '+972');
    console.log(phoneWithAreaCode);
    auth()
      .signInWithPhoneNumber(phoneWithAreaCode, true)
      .then(confirmResult => {
        console.log('confirmResult', confirmResult);
        this.setState({confirmResult, message: 'code was sent'});
        // this.createUserDatabase();
      })
      .then(() => {
        this.props.navigation.navigate('Confirmation', {
          message: this.state.message,
          confirmResult: this.state.confirmResult,
          createUser: uid => this.createUserDatabase(uid),
          phoneWithAreaCode: phoneWithAreaCode,
          signInPhoneNumber: phone => auth().signInWithPhoneNumber(phone),
        });
      });
  };

  createUserDatabase = uid => {
    const {userName, phoneNumber, email} = this.state;
    // const uid = auth().currentUser.uid;

    const data = {
      uid,
      name: userName,
      email: email,
      phoneNumber: phoneNumber,
    };

    database()
      .ref(`users/${uid}`)
      .set(data)
      .then(() => {
        console.log('New poll data sent!');
      })
      .catch(error => console.log('Error when creating new poll.', error));
  };