An important point I think you are missing here is that once the Pressbooks Book and Pressbooks Publisher themes are moved into their own releases as of Pressbooks 4.0, they will be (semantically) versioned separately. So an update to Pressbooks Publisher won’t increase the version of Pressbooks (the plugin), just its own version. Also I hope you are considering the fact that we haven’t been following semantic versioning until this point, so your past experience with breaking changes in Pressbooks is not a good predictor of future events.