I do not know of a better way using Hype's built in controls - perhaps someone else has a solution here in this regard.
I would consider another approach that does not use the timeline with its finite boundaries, because a large screen size creates a problem - the scrolling distance is set and is not going to be enough sooner or later (as we've seen).
One idea would be an "autoscroll" - when the mouse is near the edge of the screen the image scrolls in the appropriate direction.
Here's an example of what I'm talking about. While this is the not solution for You it shows the concept:
A drawback to this example is it relies on fixed zones to trigger the scrolling, whereas in a flexible environment, a fixed zone would not do - there would have to be calculations every time the screen was resized - i.e. the "auto scroll" zone is mutable.
Try a "auto scrolling" search on this Forum You will get quite a few hits.