Your point about the underlying theme or the baddies always underestimating the rebels/resistance is a fair one. It certainly fits the narrative of the saga. I could be getting a little too into the finer points saying this but wouldn't you like to see in a film that maybe just once they could learn from their mistakes? I see growing up as a theme in the new films, particularly TLJ and to me not making the same well documented errors that the Empire did would a reasonable and interesting plot/character development.
Again, I could be getting way too into this and way too technical, but why would you ever drop shields if you have them? In space micro debris would have to be a massive concern pretty much all the time. Those ships are travelling at enormous velocity through space, a bolt from one of the destroyed transports could in theory damage the hull, or worse a window. I think it would be a fundamental requirement of space travel to always have some sort of shielding.