In theory, if you think about it logically, dragons are a very cheap force, because they are assembled from the bones of dragons that died a long time ago. Therefore, I think that it is enough to simply reduce the cost of each dragon, and everything will fall into place.
As for the Death Knights, I will say this: it is useless to change anything now, but in the sixth part, I would be glad to see them in the Necropolis castle. Just not like we see them in the fifth Heroes - the horses should be alive (and not with wooden legs), and the legend will be that small horses were taught from childhood not to be afraid of any undead (by the way, this may affect their morale).
Finally, a few words in defense of zombies: they are corpses, so their bellies are swollen from gases, and they themselves are bloated.