I feel like a big part of the problem here is the total war of annihilation that all the nations of Calradia seem to be involved in. Historically this kind of thing is pretty new, and not really effective in a time of poor communications. The inability to rule everywhere at once is what gave rise to the feudal system in the first place, and even the roman empire got so big they had to split it in half. In the case of someone like Alexander his entire empire fell apart basically as soon as he was dead. To reflect that reality I think it would be great if rather than hunting each other to extinction when a nation gets weak enough it becomes a tributary or vassal. They could then rebel if their overlord was weakened or distracted perhaps. Possibly introduce some diplomatic and intrigue based gameplay for those non combat players around helping a nation to rebel/keeping them loyal and in the fold.
Related to that issue of poor communications, I think conquest could be slowed a lot by the removal of satellite phones for all the lords of the realms. What I mean here is all those popups about kingdom issues, where the lords all get to vote on new laws, I feel like they should have to take place at actual physical meetings. Ideally this would take place in the capital of each kingdom, and if you don't turn up you don't get a vote. I mean Caesar had to go racing back to Rome after conquering willy-nilly for years just to make sure the whole lot wasn't taken away by a vote. Would work best with a wargoals system I think, if not full CBs.
In my mind it works something like this: Vlandia declares war on Battania for X province (being say a town and its surrounding villages/castles). Their goal is to take that province, so they rally up an army and go do it. Now their focus becomes getting their enemy to accept peace (traditionally this is the divider between occupation and rulership). Once they manage that, the King calls a moot or an Allthing, or whatever you want to call it depending on the kingdom culture and the lords all turn up to squabble over the spoils. Once a kingdom is down to let's say its last three towns, it becomes possible to fight a war for vassalization, so that no one gets wiped off the map unless its by player intervention and again we get to create political and diplomatic shenanigans as the fallout from that war, with possibilities for quests to keep a new vassal passive, or encourage their fight for independence from the new overlord. I'm thinking this would slow down conquest, would give new importance to staying close to court, and it could be used to encourage dissent within snowballing factions. Maybe those lords on the fringe territories can't get to all the Moots and they become discontent since they aren't getting their say. These meetings would all provide a good feastlike opportunity to find lords gathered together, seek marriages and alliances, and generally do all that interesting that makes a big kingdom feel dynamic and exciting beyond a monocolour map.
I'm not sure how well I've explained it but what do folks think?