The problem is that we can lead one army and have to defend against like ~8 armies while in war with several kingdoms. But I also consider the game "won" when I´ve conquered half the map. It´s just a boring grind after that to conquer the rest.
Also I don´t really notice a huge difference between "defensive/neutral/offensive" stance if at all. It would be better if we could assign our parties and vassals different objectives like "defend area X / raid area X / patrol for enemies / train and/or build up your party".
I think the key is to either not be at war with a half dozen plus armies, or to win defensively until they can't keep going and then counter-invade. I've only played a single playthrough to unification thus far, so I'm a newbie with a very small sample size that could be the exception to how the game normally plays than the current norm, but that general patten was true to how it played for me, especially as my country started snowballing. To be frank, I really enjoy the part where you're given the impossible task of being multiple places at once and have to make sacrifices in order to achieve long term objectives, since while it was definitely painful to have to look at cities/castles too far away to continually rescue from their invaders, it was also a natural consequence of my country being too wide and once it was the strongest by a fair margin, I could no longer just deal with one kingdom at a time (it was war with just about everybody from then on). That, and I had become strong enough that winning battles wasn't a big deal anymore since I could bring a huge 2,500 troop army to stomp aside nearly all enemies, so the difficulty was strategic far more than tactical.
Still, I don't know if I'll want to unite Calradia 100% again lol, I might do as you do and just settle for half unless the other half's been united (in which case, maybe 75% or something) or something. It's just grindy clean-up once you're unstoppable and vetoing votes for peace because a weakened enemy kingdom at war is a kingdom that can't rebuilt itself to remain a meaningful threat anymore lol, especially for the homeless kingdoms.
I notice a small change; on Offensive the A.I.'s more likely to pursue and engage the small parties while on Defensive they're more likely to leave them alone. I don't know how much it affects a small unit pinning down a larger army for reinforcements to join in and clean up, but I did notice this much of a difference at the least.
However, I do agree it'd be better if we could give more concrete objectives (heck, a little system could be built around this like offering monetary or Influence rewards for obedience). At the very least, Warband's system of chatting with a guy and politely asking him to go do XYZ was make this phase of the game vastly more manageable and less RNG dependent. It'd also be a great use of clan relations (and/or possibly Influence, money, etc. to push their decision).