I don't think you understand what the problem is. There are 20 units which can at maximum go on the screen. In big battles you have minimal control over who comes in, when they come in or what order they come in. That's the problem here. I am talking about big battles simulating not 4 armies going against 4 armies because that is just the dumbest thing any military leader can do, but rather have it simulate 1 army going against 1. Only 20 units on the battle at a given time so they would have to be coming in in waves.

if you want your armies coming from the same position, position them closly on the campaign map

They come in separately. There is no option to "link" them.

but there is probably some RNG (or bugs) to simulate the fact that even the best laid plans don't survive contact with the enemy

There is no RNG there but lol did you just justify bugs being in the game to "simulate" the best laid plans not surviving contact with the enemy? Why would the epic final 4v4 battle against Chaos go wrong? It's really hard to imagine. I guess it's possible, one extremely vital messenger gets caught or something, but really, these things nearly never actually happen. There are so many safeguards in place and especially in a world with magic it shouldn't happen.