As I thought, a tutorial based approach isn't going to work. And following docs in drupal.org is not going to be a satisfying experience. Open source docs have this tendency to want to fling you to 2007, often with the direction to "Please read this" and a link to some barely related page. I haven't written any object oriented php and I don't want to get too bogged down into it immediately.
So step one, let's answer some questions: How does Drupal work? I'm less interested in about what each function do, but how does it do common things. For example, now does it create a page upon request?
Next, what does the structure of the codebase and high-level view look like? From the user interface and the application programming interfaces, down to the lowest level. How does the app work from the user's point of view? How does data flow in the context of the architecture?
I'll try to stick to tutorials and sessions, rather than drupal.org.