Please don't simplify the game!
I played the KoH 1 a lot during the past 10 years and to me the campaign auto-fights were must-have at the early stage of the game - but later when the computer also had stronger army it's a lot more fun to have more units and test out strategies when playing the battle.
1 idea to deliver happiness to both unskilled and skilled army commanders is to have a feature (hard option) to have bigger army slot - unlocking the old features for 6-9 armies on marshals. And this can be turned at campaign start => thus resolving any complains about army and battle scale.
Currently on different games (example TotalWar) up to 4 marshals can group in an attack and if you cant lead all armies there is option for AI assistance, but my point is 4 x 20 is a lot but you order them in groups(20 sec pause phase to know the environment) and the large scale battle is there + you have 7-8 groups and management is the same as having 5-8 armies in total BUT with exception that you can change one unit from the back line to round the enemy if they let you => this brings diversity and deeper game strategy feeling. Having only 8 units and attacking with solo marshal (if someone quickly expands on many fronts) will lead to no large battles and the scales of them in the ages were quite big.
Hope you find anything useful and keep up the good work!
I think many people will agree that mildly-higher difficulty won't harm the newcomers to the genre and the old KoH had big complexity for it's time. Replay value suffers if the game is more straightforward and simple.