Integrate IdentityServer (or any other IDP) out of the box by using KV/D1 instead of Redis/SQL Server

IdentityServer won’t charge you for the first $1m:

For-profit companies/individuals with less than 1M USD projected annual gross revenue and with access to less than 3M USD in capital facilities
Duende Software

It’s the best .net IdentityServer there is (that I know of).

Or integrate any of the certified oidc providers:

Such as this java script one:

The community is already making some CF Workers integration with Firebase Authentication:

But there is really no need to host this is Google, you should be able to host this in CF.

Also Firebase Authentication would charge you 0.0055 * 99.999 + 0.0046 * 900.000 = $550+$4.140=$4.690 a month for the first 999.999 MAUs (which is a lot!):

Identity Platform charges per Monthly Active User (MAU) for most sign-in methods. Any account that has signed in within a given month is considered an active user.
Precios  |  Identity Platform  |  Google Cloud

CF Zero Trust is more for employee web apps (not customer facing web apps).