It's not clear at all that a scammer is on the phone, instructing people to click through every warning that they see while sideloading a malicious app. As I stated up thread, the majority of these scams are happening through apps in the Play Store.

To address your question, there should be a straightforward option during device setup. If you're first attaching your account to the device, you simply check a box that says this is an advanced user's phone. You can put it behind the same kind of scary pop-ups that web browsers have when they're about to serve you an HTTP page, or when the HTTPS certificate is self-signed.

It's the most obvious, straightforward, user-friendly approach, and it was never even discussed.