Just a few suggestions:
Have Battle Phase 2 unlock both the T3 and T4 buildings.
One building will contain Stug, Rocket Artillery, and Brummbar.
The other will contain Ostwind, P4, and Panther.
Make Stug slightly more accurate against infantry.
Increase Panther damage to 200.
Battle phase 3 will unlock Brummbar and Panther.
Battle phase 3 will unlock the reduced reinforcement cost which is now tied to the HQ (increased cap rate is removed).