Couple things. First, why does no other Imperial notice a gigantic space hulk coming at Armageddon and not do a thing about it? The Imperials have a massive war-fleet around the planet. I'm sure they would have done something. Second, a Baneblade uses solid munitions for its primary gun, so there is no charging it up. Perhaps you were thinking of the Shadowblade?
being reeled backwards, its length might actually reach to the outward side of the gravitational pull of planet Armageddon
Umm. Allow me to put it like this, 400 KM above the Earth, the gravity effects the astronaughts get is 90% of Earth Standard. The Moon is within Earth's gravity pull, that's 382, 260 KM away. Space Hulks are big, but they are not that big. Assuming that Armageddon is about Earth's gravity, we'll assume the hulk 400 KM so we can assume it's butt is out of the full strength of Armageddon's gravity and thus, the dead orks can float about a bit in space. So, you have a 400 KM long space ship slamming into a planet. Now then, what would a ship like that do so a planet? Well, in real life, the space object that may have ended the dinosaurs is thought to be about 10 KM long. In 40k, we have a hijacked factory ship slamming into a planet and altering the landscape of the majority of a whole continent.
Plus, I thought it would ad more emotion to the story from the beginning
I'm sorry it does not. We don't know who the tank's commander is, we don't know what he looks like, his personality, or even his name. We only know it's a guy because you used the pronoun 'he'. I'm sorry, but no one will be invested in that character, and since no one is invested, there is no tension or emotion added by said nameless tank driver.