This may be too much for v5.7, but what about changing the way brigades and divisions are organized. Using a similar technique as the task force organization, this would allow more flexibility in the arrangement as not all countries follow the 4 btn/bde, 4 bde/div model.
Making a Corps, or Army, headquarters would also give the top field commander somewhere to go. Right now in my game I have Colonels commanding Regiments (Battalions), Brigadiers commanding Brigades, and Major Generals commanding Divisions, with three Field Marshals commanding the senior division on my most heavily populated planets. Having a Corps, or Army, Headquarters Battalion, along with the reorganization of ground units above, would allow Field Marshal Killemal Alyens to command the entire invasion army of six divisions.
Just a thought.
Adam.