HTML form, JS and Firebase

I am building an HTML form where I add a user to Firebase. The fields are first name, last name, phone, username and sponsor. First we do the Firebase OTP to check the phone number, then we need to check live if phone and username already exist and if so, we need an error to show. If ok, then we create the user in Firebase. Then we have the Stripe payment and then confirm page.

Is this something you can help with? Please only give a bid if you have done live database check and Firebase insert from HTML form before.