Firebase Auth

Hello.

Context:
I use Squarespace to make landing/launch pages for my apps. I usually create a launch page "someexample.com" (Squarespace) and my app lives in the subdomain "app.someexample.com" (GCP) with a backend handling auth at "authbackend.com" (GCP).

Squarespace customer account credentials are stored in the www subdomain-- "www.someexample.com".

I would like to sync customer accounts with my firebase auth & authz. Are we able to use the "/api/site-users/account/session" or "/session" endpoints which are created when our site is created? Or use the same api on our backend?

There's some sort of api to retrieve customer emails but that doesn't meet my requirements.

Do I have to forfeit all the good stuff Squarespace offers (digital storefront, plugins, etc.), just to use firebase sso?

These are the current options as I see them:
1. Create a second login prompt on the launch page someexample.com using Firebase Authentication
2. Create a second login prompt on the subdomain app.someexample.com using Firebase Authentication
3. Disable customer accounts on the frontend only have a login on the app subdomain to reduce user frustration
4. Host the entire project on GCP