Thanks to @brady.miller, the docker setup is significantly easier and has allowed me to test my theme workflow in a working development application.
I filmed a video of a trivial change to show how I will be approaching de-dupeing a lot of our theme files: https://drive.google.com/file/d/1HoGoWNOcO-t7oiwVFzWxlskbhz2oilko/view?usp=sharing
Next step is to get to a consistent set of base themes and overrides (similar to the core.css imports we currently have) so that we don’t have three deviating themes. A PR on this should be on its way this weekend or in the coming week.