Far as I'm concerned, Boyd nailed it in post #4. It's sad, but with sandbags, it is my experience that "returning to battery" is in conflict with "shoots smaller groups."
Try this. Once you've gotten a bit of sand out of the rear bag, go ahead & slam it down on the bench to get it flat. Now squeeze the sides of the base a bit, to "fluff it up"; to get things little loose. After each shot, again squeeze the bag to come back close to the original elevation --close is fine, finish getting on target with the joystick. (Or just squeezing, if you're up for that...)
I'll bet that if you fire a couple 10 shot groups this way, and another couple 10-shot group as you're now set up & shooting, you'll find the fluffed & refluffed bag shoots smaller. Not quite as quickly, but smaller.
Over time, (1) you'll get the amount of sand in the rear bag base & ears about right, & (2) develop an instinct about how much to "fluff" things, so it won't slow you down much at all.