Yeah, something that's not actually shown is that changes to the optimal mass of thrusters also change the maximum mass.
It looks like those are 6As, and you'll be pulling the maximum mass down below the laden mass of the ship if you try to engineer past dirty 2. You can compensate by using the Drive Distributors experimental, which will let you go up to Dirty 5 (Maybe 4 depending on the rest of your fitting), or you can use bigger thrusters.
It'll still fly like a dead donkey though.