I'm talking about difference at one battle. For example I tested different unit compositions at Cross Keys (Confederates). When i brought only 4 infantry units AI decided to make deep flanking from North and South directions. Also he just charged at me from the start. When i bring 6 infatry units AI cames back to classical strat - take middle and slowly approach across the river. And i had the same thing recurring at different battles (for example Battle of Chantilly/2nd Winchester).
No, I didn't add any men to units after farm part.
It's BG diffculty.
First of all i checked Forefall campaign, you mentioned before. He noticed that agressive aproach to 2nd Bull Run makes it much easier. So i decided "what if I will do the same thing, but even more agressive and do better micro". General setup was: infantry units 1 100, arty 4/6/8 guns (depends on amount of guns), ranged cav 500, melee cav 750. I definitely reduced Union forces by splitting my reserves to 2 parts. 1st reserv corp was only 3 cav units, second some art, inf and skirmishers. I have other campaign where different composition of reinforcements increased Union forces by almost 15k.
I decided to take fortifications at the middle between forest and railroad. At couple of first tryouts i had normal losses at northern part of map, so i decided to concenrate almost all my artillery here (except of 2 artillery units). It were 24pows, 20pdr parrot and some rifled arts. I had 3 cav units from start (melee - lemats, ranged with maynards and sharps). One of them is 3 star unit from Shiloh, i got lucky and rolled all 3 ranged perks here. Second also 3 star range cav with maynards.
Basicaly you know that Union forces are coming from 3 main directions - north, west and south. So you bring you cav at center around vision point and wait till spawn. You should also send some dedicated skirmishers to the forest at north part, south vision point and at forests at the middle. You need them just for vision. Tactic is simple - skip spawn of nothern group and let them almost approach woods. When attack slow arty with cav units, fall back. Do the same trick with middle group. So 1st group is freezed around woods, 2nd decides to stop near the wheat field/farm. Send one ranged cav to south group of units, kill arty and take agro from infantry. And other ranged cav plus melee cav should go to the river near the city. Hide melee cav at small forest and take agro from Union cav on hill by range cav, fall back to the city direction and fire when Union cav will be crossing the ford. Charge them by melee cav then they will low on morale. After that fall back, so southern Union group will freeze between forest and city.And go for nothern group. They probably would come to the woods. Till this moment you almost don't need to control all your army except 3 cav brigades.
Don't fight with skirmishers you need them just for vision, so you arty could fire from max range.You should bring all three cav units here when Union will decide to charge at you. Kill remaining arty and start flanking fire on infatry by ranged cav to rout them one by one. You should have enough time.
I think that AI logic breaks somethere here. Agressiveness of southern group seems to be tied to attacks at the middle, so they just stay at the forest near Groveton almost till the end. They moved out from forest almost when i got reinforcements. At the same time agressiveness of middle group breakes if you attack them at wheat field and continue to harras them here. While nothern group is so harrased by cav and arty, so they just coming under fire and rout before reaching your defense lines.
Main trick here is simple. Don't even try to charge with cav and forget about rifles or other weapons. You need sharps/maynards only for morale damage. Split you cav, so one will fire from rear, one from flank. Then enemy brigade will rout bring your melee cav with lemats (lemats is a must for higher melee damage, so your cav will spent less condition) and switch ranged cav to next infantry brigade. So your melee cav will attack routed units and won't take losses. After that just look out for outside units and do the same thing. Stay with melee cav near enemy (let them rest) and just take down routed units. 1st ranged cav take agro, but you should continue to move around, so infatry will try to rotate to the left/right to make a volley, while 2nd range cav firing from flank/rear. 2 and 3 star range cav spends almost no condition on moving, so they are over 85% almost all time. Just leave one or two captured supply wagons somewhere near center of the map, so cav could refill their ammo. Melee will provide a lot of captures and early shatters. Basicaly you fight morale threshold to shatter/capture.
You can practice carousel trick on charged infantry units. You easily see which cav unit is the target of charge and just kite back. Infantry strangely won't change the target at 95% probability. While other range cav doing flanking/rear fire. When infatry's flag start to throb just send in melee cav. Yeah, it's really micro intensive and requires a lot of your attention. But you can do it because you almost don't need to control the rest of your army.
To cut a long story short, did the same thing with all enemy groups and get them one by one with cav. You just need to force them to freeze at the start of the battle. It's very similar to Antitaem. But it's like having three separated mini battles instead of one big battle.
Thus my main cav units finished 2nd Bull Run like this, while enemy had almost similar amount of killed/captured.