Hi,
I have just read your description and I am very impressed with you. As a freelancer on freelancer.com, I haven't seen a client like you . I am a talented PHP developer and have an extensive background in firebase. When I have to develop serverless apps, I used firebase and made login forms with Firebase Auth Key. As a senior WordPress developer, I have highly experiences in integration API. So therefore, I can easily accomplish your desired goals within your timeline.
Let me know if you are interested, I can provide you reference as well.
Regards,
Chunyan