Nice shot, Alan
If you can handle 5 min at 2000mm, try extending the exposure time up until you find your present limit, then back off a little. That way, you might get sub times up to 7-10mins and be comfortable in taking pics at that focal length. Then begin the process again and extend your sub times once more. That way, your images should improve as you learn to grab more photons, and also as your processing skills improve.