I think examining other patterns would be useful.

This makes me think that the problem in the SO question you answered for me (posted under my old name) (https://stackoverflow.com/questions/76213875/fanning-out-and-summing-results-in-rxjs) could potentially be more efficiently handled with expand(). There are a couple of cases where I'm fetching all of a particular paginated resource using expand() and reduce(), but from what you've written here, I think I could use expand() to create an observable that emits each page of the resource data and chain further functions to operate on those pages.