Hello!
First of all thank you for very detailed specification, not often here publishes projects with such a good description.
But one important point remained unclear for me. Do you want to use GCP (Google Cloud Platform), SaaS/PaaS/IaaS? Or you want "standalone" web service (front-end, back-end).
After I receive an answer to this question, we will be able to discuss the implementation.
Thank you for attention!