I think that ext.db stuff is just a wrapper around Cloud Datastore. As for the task queues, they are almost identical - see Google Cloud Tasks (https://cloud.google.com/tasks). You might be thinking of google cloud pub/sub as the completely different one.
The way we did the migration was to first move our application code to use the first class google cloud APIs and not rely on any GAE standard libs. Once we got to that point we moved our application code to GKE. Not trying to make it sound easy, it was kind of a pain.
I was specifically talking about push queues, not pub/sub. My comments are also based off of migrating from Python 2.7 vs. 3 where there are additional migrations challenges and GAE limitations.
At least in our experience, it was (or will be...we'll see) easier to migrate off to a generic container based environment.
push queues are available in Google Cloud Tasks. We use them. There's even an article about how to migrate from the appengine ones to the google cloud tasks one: https://cloud.google.com/tasks/docs/migrating
The way we did the migration was to first move our application code to use the first class google cloud APIs and not rely on any GAE standard libs. Once we got to that point we moved our application code to GKE. Not trying to make it sound easy, it was kind of a pain.