Commanders should not cost anything. These are nothing but remarkable Generals who fought either in the last war or were just full of advanced ideas and knew how to fight using the tools of the day.
They got to give units some advantage of sorts either movement and or combat. Usually, Generals tend to use the tools of warfare in a unique way. Most will benefit with tanks, while others with infantry or artillery.
Perhaps we just drop all the combined arms bonuses and require that the commander facilitate this and invent new combination’s of units.
Or alternatively, the General could just be its own combat roll ( say at 4 or less) or a re-roll of a miss
Or make a list of generals, where each has its own unique bonus to units and by type and employ them by front.
I do know that Infantry don’t need any boost any more than what they currently have. It seems that tanks are the only candidate for some type of boost since these are pricey.
A movement bonus for a General seems to be in order as well.
The most simple KISS is to just make them another unit rolling at 4 or less on land. That would minimize the disruption of the games math. But it seems really too simplistic and bit on the shallow side in terms of modeling some flavor of WW2 warfare.
Perhaps generals can be assigned a leadership rating say 5 and you now got 5 points to use as follows:
use 2 points to move infantry 2 spaces
use 2 points to boost 2 tanks +1
use 1 point to retreat one tank when the enemy counter attacks you
Spend all 5 points
Each turn you get another 5 points to use as you wish
you can either boost a unit on attack or defense, or move a unit +1 space….all using one Leadership point.