Hey Bob (and others),
It's a great thread. I have a comment and request about that from the Airflow Committer and PMC member point of view - and someone who has an interest in both - development environment and Composer alike. And I would love to hear more people commenting on that.
I created (over the last few years) Breeze development environment for Apache Airflow development. The tagline for it was "It's a Breeze to contribute to Apache Airflow". But from your comment, I have a feeling that we might need either something similar to have the tagline "It's a Breeze to test DAGs for Composer's version of Apache Airflow" or even (and better) more general - "It's a Breeze to replicate ANY deployment of Airflow locally to test DAGs in it".
It's much more for the development of the Apache Airflow itself rather than for DAG development and testing, however over time, we have used it internally for automated system tests (mainly for Google Cloud Operators). Many other people used it for testing their own DAGs as well and it already contains a lot of useful stuff that you already mentioned - for example running it with a MySQL backend is literally adding `--backend mysql` and optionally `--mysql-version`. We also use extensively some parts of Breeze during automated CI tests of Apache Airflow. It is intended to run fully locally (so no Cloud SQL, or GCS or other external components that the Composer uses are there). However, I think (please correct me if I am wrong) it is mostly about PIP/APT dependencies synchronization that you actually miss currently - not the other Composer/GCP-specific "external" components?
I wonder if you and others could take a look (try and see) whether Breeze is a good start for something similar, what you'd miss if you would like to turn Breeze into a "DAG testing environment for Composer" and general feeling about a tool like that. What features you would like to see, what you currently miss, etc. I would love to hear more and gather some feedback here - and possibly bring it into discussion to the community of Airflow and to the Composer team.
J.