Two pennies worth.
I have coversed with Sergbutto and apparently the vulnerable spots for this ship are the same as the commerce raider, including the magazines (think of it as carrying explosives).
After having occasion to reset my game, I have now altered the large merchant to have a tonnage of 9,600 and a renown reward of 270. This is a personal choice, but I just found it too big at even the reduced 11K+
