Sorry, it may not be the right word. I mean the effect that clicking on the little "[-]" does. A way to automatically clic this for all second-level comments, so that only first-level comments that directly answer the "Ask HN" appear.

Got it. I think this would be useful. But where in the UI would we put all these options? This is my main concern.

If you find a place for comments sorting options (what my parent comment initially suggested), it could go there too (it's also related to comments display settings).

Maybe a line between the "add comment" button and the first displayed comment?