I have an idea for the campaign recruitment system. There are two systems I thought up to mimic the levy military system of the time. They both require modifying the way replenishment works and by what I have seen with DIE most of it should be possible.
First system is to remove replenishment from all levy troops. So a band of peasant spearmen will have its casualties kept permanent until it is disbanded and another unit is raised. To balance this out you could make upkeep for these units incredibly cheap. This system means veteran units will most likely have very few men and merging injured units will be far more common.
If you wanted a system that would allow a unit to keep it's veterancy then you could just make it that units cannot replenish in certain seasons. My personal idea would be to have all seasons apart from autumn (after farmers have finished harvesting) blocked from troop replenishment. Then in autumn replenishment is allowed and accelerated so even a unit with one man will be reinforced completely in that one season. This would mimic the way that men would go to war during this period after harvesting their crops.