With over 1,100 sq ft of cooking space, it’s no wonder you have a problem maintaining a temp!
one other question - what are you using to validate your temp? if you’re using the built in themometer - ‘don’t’ - those are notoriously bad - get an oven themometer and check at the grates.
Also, are you filling your firebox full of coals? Give me an idea of how you set up your fire. Also, as said above, take an oven thermometer and take a few readings. One where the firebox meets the cooking area, one in the center, and one right under the chimney exhaust. Just want to get an idea of what the temps across your cooking surface are. (ETA: And, yes, your thermal mass/heat sinks/bricks would go into the main cooking area.)
hey - it uses TARDIS tech!