I would suggest to you to use service account credentials, it is much more efficient to authnticate between BE and google API, specially as it is not deferent accounts( users ) what you need to authenticate to
you can follow this processes
https://isd-soft.com/tech_blog/accessing-google-apis-using-service-account-node-js/
is it private project what you are working on ? I will be interested to know more about the project if it is a possibility
thanks