Make 2 different HQs and 2 layers of formations to begin with. It's what I'm doing in my 1890-campaign to maintain my sanity with 10 nations.
The basic building block is the battalion and each battalion is built to do one thing out of three:
1. Attack
2. Defence
3. Support
These also match with the positions (FA, -, SP) Aurora allows for ground combat.
Attack (Cavalry) battalion has LVH HQ and a whole bunch of LVHs armed with CAP and LAV.
Defence (Infantry) battalion has STA HQ and a whole bunch of INF armed with PW & CAP & LB plus some STA with MAV.
Support (Artillery) battalion has STA HQ and loads of STA with MB.
Then I make a Regiment that's only a big STA HQ - but eventually, I will put HB or LrB on it as well so it can fire from Rear position.
Now the Regiment will have 4 battalions under its command: 1x Attack, 1x Defence, 2x Support. Once I have HB in the Regiment itself, I'll take the second Support battalion away.
This way each regiment is a self-contained fighting unit and you can just add more regiments as needed for an invasion. Attack battalions are on Frontline Attack, Defence battalions are on Frontline Defence, Support battalions are on Support and the Regiment itself is on Rear. This way all units maximize their usefulness vis-a-vis fortifications, breakthroughs, evasion, and supporting bombardment fire.
Once your empire grows, you could switch from battalions to regiments as your basic building block to avoid micromanagement hell.