This is cool - Whenever I have a new idea for a thing I spend too much time writing boilerplate IAM and backend stuff, taking away time that could be spend on actual business logic. Thought about packaging the boilerplate stuff up before, never gotten around to it. Glad you did!

A thing to consider would be to make it easier (or perhaps bake it in) to separate out parts of the app into a separate origin. Something that would be good for pretty much any SaaS app would be to separate the IAM out (could still embed it with an iframe) - this allows you to keep a fairly tight security policy for the IAM stuff and a more lax one for the rest of the app. Kinda how Google separates out accounts.google.com.