If you ever have to enable public OAuth on such a project, you'll need to provide a list of all the API projects in use with the application, and Google Trust and Safety will pressure you to merge them together into a single GCP project. I've been through it.

You can do what you're describing but it's not the model Google is expecting you to use, and you shouldn't have to do that.

It seems what happened here is that some extremely overzealous PM, probably fueled by Google's insane push to maximize Gemini's usage, decided that the Gemini API on GCP should be default enabled to make it easier for people to deploy, either being unaware or intentionally overlooking the obvious security implications of doing so. It's a huge mistake.