have you even considered maybe the Northern Empire and Sturgia are just weaklings that aren't fit for survival in the lands of Calradia?
who cares if they get destroyed?
like in my 1.5.3 playthrough, i worked as a merc for the first 4 years, in which time, not a single town/castle was lost by a single faction (i basically played as "referee" and made sure any siege is defended or countered later, if the Khuzaits took a town from Northern Empire, i joined NE and took it back. for every settlement and every siege)
so when i finally started my own kingdom, every other kingdom's power was relatively balanced.
then i noticed the Khuzaits dominating the NE again. and i just took those towns and kept them myself. i went to war with the Khuzaits for months, with my mercenary clans in my army. i killed 5000 of their army maybe more. captured 30 of their lords, they escaped and i captured them again. i'm talking constant spanking. decimation of their forces. using merc parties to raid 20+ of their villages. all the while the northern empire is just resting, gathering forces and recovering. After all that, i was like, maybe now the Khuzaits are weak, the NE might come and take a few of their towns. i proposed for peace and took the 2000/day tribute.
literally 1 week later i see a 1500 men northern empire army, i was like wow nice maybe they'll siege Makeb or something. later, i see notification saying that army was wiped out and all the lords got captured by the Khuzaits, i literally destroyed all the first round of army the Khuzaits had, and the second round. by then they should have only tier 1-2 recruits yet they still managed to destroy the 1500 man army led by Lucon.
when i see the Khuzaits in a losing battle, they just sacrifice a few men and escape. their map movement speed is too high