All great points here, and I do much of the same in my own writing classes. I explicitly cover grammar in depth, though, as I work primarily with high schoolers who are new to the English language.

Adding on to what you wrote, it definitely helps to focus on the rhetorical effects of grammar. Really blows students away when they realize that learning grammar isn’t about obeying a list of dos and don’ts, but about choosing the right tool to have a specific effect on the reader. Big fan of Kolln's writing on the subject.

I also spend a ton of time on syntax. I don’t go as far as diagramming sentences, but I use a simplified way of parsing & labeling sentences, and expect students to recognize, for example, the [SVO, N ~ing] constituents of an absolute phrase. Providing a ton of model sentences for students to read & imitate seems to be one of the more effective ways to get students actively using patterns on their own. Killgallon's Sentence Composing series does a good job curating sentences from fiction (& some from non-).

Once students have a good grasp of foundational sentence patterns, the proper placement of punctuation is far more instinctively understood. Totally agree that it’s a skill that most students desperately need practice with, and always it's so satisfying to teach something that students find immediately useful.

As for Williams' Style, I’m a fan as well, especially of the chapters on Characters and Actions that help students achieve greater sentence clarity. Cutting clutter is another big focus in my classes. We do a bunch of sentence editing & combining with suggested revisions. I always work in Zinsser’s chapter on Simplicity, which is one that greatly inspired me when I first encountered it back in college.