Only using the official API is quite cumbersome, as fetching a story with comments requires one network request per each comment.
Instead, try considering the (more unknown) Algolia API[0]. It supports fetching the entire tree in one request. Unfortunately the comments aren’t sorted, so you’ll still have to use the official one (or parse the HN HTML) if you need that.
For my HN client[1], I parse the HN HTML (as it’s required for voting), and sort the comments from the Algolia API. It’s much faster than recursively requesting the official API.