Joel, I’m a huge Pressbooks fan, but one of Pressbooks best features is that it is linear and straightforward like a book.
When I am onboarding a new project, one of the things I often ask at the beginning is if it is linear or not. 90% are meant to be used as books, which makes Pressbooks a perfect option.
If you were my faculty member, and said that you needed a non linear project, where certain categories of content might be used over again, with reusable content, I might actually push you towards using vanilla WordPress for the project.
One of the powerful features of WordPress that is deactivated from Pressbooks is its powerful categorization system which allows content to be categorized and tagged, and then reused throughout the system. In your example, you want to be able to categorize some examples as calculus and some examples as probability. If you built the examples as WordPress posts, you could start to categorize them, this ones calculus, this ones probability, this ones neither, this ones both.
Then as you were creating the page, you could start to use query loops to display those posts. First give me 5 examples of probability that do not have the calculus tag as a warmup. Then I have my lesson, then give me 5 calc examples, then give me 5 examples that need both probability and Calculus.
The trouble with this solution, is that you would spend a lot of time recreating what Pressbooks does best. You would have to manually create a landing page that helps users understand your content and navigate. You would have to manually create a content view that includes a navigable, accessible TOC on the left side. You would have to insert the next and previous links for going content area to content area.
But to me, with the example you show, what you really need categories, tags, and maybe a few additional ways to categorize your media, perhaps by installing a custom taxonomy.
I do this at a very low level of categorization on my personal website ed-beck.com. All of my blog posts are show up as Projects, but then I categorize in multiple ways. I built an unused prototype of a site that did a lot more categorization and filtering at https://sunycreate.cloud/showcase/. If you look at it, I built 5 different ways content could be classified, each one was a custom taxonomy. I used a plugin (Taxopress) so that I could manage all those custom taxonomies from a simple interface. I installed a plugin to search and filter all those taxonomies (Search and Filter Pro), and I used a WordPress Full Site Editing theme so that I could modify the search template, the index template, and frontpage without having to do any PHP code. All customization were done through the user interface, without any coding.
I don’t think what you want to do is doable through Pressbooks, but it may be through a more vanilla WordPress with a few extra plugins…