The reason for the Stub P4 is to let it be in T2. It allows OKW to have a tank with strong AI capabilities, like the P4 already has, but prevent it from being able to hurt heavier tanks. A Panzer 4 in the Flak HQ probably doesn't work with stub.
I assume the stub p4 would just be an AI version of the p4 still with medium armor and health, because the luchs is also in that same tier. If so then great job, however with rear armor being perticularly weak now you could run into t2 into stub p4 spam if the proper care is not taken with balance. Essentially the problem with Stug Es now except for the fact they're not callins. May just be wiser to place on the t3 and keep it available prior to flak HQ/panther upgrade.
Also you do have the MP40 volks upgrade and maybe an STG which is fine, but if volks remain at the 250 manpower mark and their manpower to power ratio remains the same, higher manpower CQC units may run them over. Especially in map dependant scenarios.