Having a main sail that acts similar to yours, I'll tell you what I have found. My sag showed with creases at the front of all the battens, not just the lower ones. Yes, the sail is blown out, but you can cheat a little. One thing I was able to do that you can't is add some pre-bend to the mast. This reduces the fat belly from being blown out. This helps a little, but I'm not sure how you bend a masthead rig, especially of the spreaders are not swept back. Secondly, since by your description, your boom slides up and down, I would mark where it is the last time you raised the main and pin it, or tie it down. The next time you raise the main, use the winch to tighten the halyard. Crank on that puppy. Lastly, rig yourself up a Cunningham. That will twist the triangle shape and put some tension on the belly. The tension will not help the lowest part of the belly, but it will help the middle. Once you get all that, then adjust your main sheet to get all of your telltails flying. If the top leeward telltail is not flying, move the traveler to windward. It isn't going to fix it, but you may be able to make it better.
I will reinforce other comments. You have to stick telltails on your sails, otherwise you have no idea what the sail is doing. I figured out how to get most of the sag out of my sail, but at the same time, I lost speed. I finally stuck telltails to the sail, and could then finally see the finer aspects of sail shape. I can tell you that what looks right as far as the sail shape only puts you in the ballpark so your normal tuning things will work. You really don't know without telltails on the sail what is really happening.
Oh, something else I forgot. I also loosened the tension on the battens slightly. that also helped.