That's fine during peace-time. If your realm is locked in a life-or-death struggle with another realm of approximately equal strength, letting your Marshals debate on turn-by-turn decisions is a fast way to get your realms killed, imho. The more armies you have, the more chefs end up in the kitchen and the more things can go horribly wrong.
If you trust your marshals that little, your realm ought to die.
If your general thinks that he is the only one who can properly lead the armies, your realm ought to die.
If you need the general (maybe we need to abandon that term, it really is more like a minister of war) to issue turn-by-turn orders, then your realm ought to die.
You are complaining because you are trying to have the general do what is actually the marshals job. They have the information needed for turn-by-turn detail actions, so why don't you let them do it?
The general is supposed to tell his marshals to defend region X, attack region Y and make sure that there are enough forces near the border to Z to repell any raiding attempts.
If you act any more detailled than this, then you are not playing a general, you are playing a marshal who isn't happy with just one army.
And the "one big army" strategy isn't a solution, it is a big part of the problem. With a large army, of course you will only ever have a small part of your forces in any given region. Try smaller armies and you will find they are much more responsive and unified. Nothing in the game forces you to put 20 people into an army. Have you tried 5 ?
