It has pretty much been summed up already, To take it to the next level requires alot. Back tension can and will improve you the most with the least, but from there comes a BUNCH of mental training. It all depends on what you want out of archery. My family up and left partially because of my obsessions. I now have lots of time to kill and shoot about every day year round. Archery is not really a sport or a skill, it is a discipline, it is about mind control.

Sounds like you are shooting pretty good though. A good lethal group at 20 yards is a consistant 4 inches. You seem beyond that. Good luck to you this season.