Hi Savannah,
I think "book" is the correct type of document for this (article could also work, but I like thinking in terms of chapters for main divisions and sections for subdivisions). The course template that regular pretext has is secretly a book too (we have just hid it well :)). So I think you are on the right track with what you have.
I'm planning on coordinating Calculus this Fall using PreTeXt.plus myself. There are only three sections (me and two graduate students teaching one each), so what I'm planning on doing is having a common syllabus with different build versions for each section. We will all collaborate on the document, but only edit our own "components" for the customizations for our section (like instructor name and student hours, etc). I don't know if this would work for more than a small handful of instructors though.
I think what you will probably want for sharing is to get your base course set the way you want and then let each instructor create a copy of it. That means that they won't get updates after copying it. It wouldn't be hard for instructors to go back to your copy and actually copy/paste new content when they need it though.
As for sharing just parts of the course "book" with students, you have a few options:
1. If you build the entire book, and set the build settings so you have one-page-per-section, then you can embed just the page of that section in your LMS. As far as students know, there is no other page shared with them; they never see the entire book, only the embedded pages. Technically they could find the whole thing if they inspect the source of the embedded code, so I wouldn't hide exam solutions this way, but for suggesting which parts to look at when, I think this is very effective.
2. PreTeXt has the notion of a "version" of a book. You specify which components you want included, and components that don't match are dropped. Note that PreTeXt.Plus does not yet have this feature in the build settings, but it is the very next thing I plan to add (later this week).
3. Like you said, you could have multiple projects. I'm planning on having a student-facing project and a separate assessments project (holding the quizzes and exams) which I will never share with my students, for example. I know other instructors do this with a separate syllabus. I think it is perfectly reasonable to have separate projects for roughly each chapter of the sample course you started with. The disadvantage of this is you cannot easily cross-reference things in different parts then.
Does that help? I'd love to hear about what you settle on. We can implement features for PreTeXt plus that would make it more useful to your use case.
Cheers,
Oscar.