I have to disagree with you on the Panzer IV. That was actually a pretty sturdy tank and I think comparable to the Panther (Panzer V). They both had 75mm main guns, although I think the Panther’s had a longer barrel.
The Panzer III on the other hand was closer in comparison to the Carro Armato M13/40. The Panzer III had a 50mm gun while the Carro Armato had a 47mm gun. Armor was roughly equivalent although I think the Panzer III had better performance.
Also, the WOTC OOB tanks are some of the best medium to heavy tanks from WW2 – T-34, Panther, Sherman, Matilda. The exception being Japan as they went with the Type 95 which is a light tank.
Therefore, for uniformity sake, I think FMG should make the Armor #1 (tank) one of the light to medium versions for each country and maybe save the heavy tanks for a later set.
Russia = T-26
Germany = Panzer III
Britain = Crusader
Japan = Type 97 Chi-Ha medium tank(since they already have the light tank)
USA = M-3 Stuart (nicknamed “Honeys” by the Brits)
Italy = ALREADY DONE
As for SPGs, I’m not as knowledgable about them. I know a few of the German pieces – Hummel, Elephant, StuG III. I’m also familiar with the American M-7 Priest and the Japanese had one called a Type 1 Ho-Ni. I don’t know about British or Russian SPGs.