I've used GAE quite extensively at my last place of work and really liked it overall. I experienced some large pain points when I was unable to use libraries that used C extensions, which I guess should be solvable now. The GAE runtime was (is?) also only available for Python 2.7.

I wonder if there is a future possible migration path from the standard environment (Classical GAE) to the new flexible environment.

For our product we also decided to use the Google NoSQL database Datastore. Since this is/was tightly integrated in the GAE Python runtime, with its own data model layer, it was an easy choice to implement. But, of course, made migration away from the Google Cloud really cumbersome.

I guess you still are limited to use some of the Google Cloud Databases, but if you chose Cloud SQL maybe the migration is more feasible?