I'm not really sure why you think it's Amazon only or a walled garden. I have limited experience with it but from the research I've done thus far it seems to have a fairly robust development platform.

I'm under the impression that if it feels Amazon-only it's because of a failure of companies and devs to take advantage of the platform, not a failure of the platform itself. For example the Google services you're asking about, is that Amazon's fault or has Google not taken the time to develop that app because they're focusing on their own product?

I've used my echo dot most often with 3rd party services so far (daily briefing integrations, capital one, yahoo fantasy football).