I am proposing a mechanism to solve the issue of marshal counts due to the limited no. of slots in Royal Court.
Apart from marshals in royal court, all cities must have up to 4 slots for Army Generals to be hired; One for the defense of City garrison, One for the defending mosque/monastery, One for the defending farm And one for defending coastal village/port in case of raids. Player/AI should recruit separate armies for each generals just like they do for marshals.
The difference between marshals and generals is that generals should only be able to defend lands in case of siege/raids while marshals can do both, attack enemy lands as well as defend their own lands along with generals. Player can not control movements of generals army on the map. In case of raids, respective general army will automatically move to the farm/monastery/coastal village being raided where they engage battle with enemy then player can control the army in the battle just like any other marshal army. After battle, if successful, general army automatically moves to the respective city. In case of unsuccessful battle, slot becomes empty in city slot and new general should be hired. This concept share its similarities with the jihad mechanism already present in the old game but with advanced work around.
This idea makes sense because the major issue with having limited marshals is to defend against raiding. It becomes difficult to impossible for anyone to defend their lands when kingdom becomes large specially farms/mosques/coastal villages.
The idea of non-marshal knights of the royal court leading army makes no sense at all. By the way who have seen diplomats/merchants leading armies to fight enemies in the battles in history? This game is great prospect. Don't spoil it.