How to insulate under a front door
That opening will be sealed off with a riser.  Make sure it's a tight fit with no gaps.  The horizontal pieces extend farther than the framing, so add wood to extend the vertical framing to make a 4-sided box that the riser mounts against.  Caulk between the riser and that threshold "box" to seal it.

Beyond that, fiberglass batting like you show should be fine to fill the space.