I believe it would be an improvement if you could merge single units [1] into parties [4] and parties into squads [8] and a squad and a party or three parties into company [12] (or three squads into two companies). You should only be able to do this in towns that are able to produces them. When you combine them the xp gets averaged out less one third and it takes a season or two.

It would also be good if you could upgrade units. Every time you upgrade a unit it loses one third of its xp, so you have to chose between the loss of xp and upgrading its equipment.

I believe that having to train up new units all the time slows the game down.