This will take lots of time to grow. Skip t5.. Open 30 dorms and fill them with 30 t4 and upgrade to level 4. It will give around 330kcs+ from here directly get t6. You will have t6 level BC in just 2-5months
T4 level 4 decrease plunder, and it takes a long time to save up to 10b from t4 l2, saving up to 40 would take even longer.