IDK why you skip few facts
1. medium doesnt mean P4. Stug g is cheaper and its medium
2. OH can get 1st medium as call in
3. you skiped sov T1 which means playing with disadvantage
The stug is used as a stop gap AT unit, rushing it makes no sense unless you are incredibly behind, in which case you're better off just playing T2 stall into P4 command tank with pak support for AT. The stug compares more directly with the SU-76, which comes earlier as well.
Yes, the Stug-E is a medium call in, but this is CP dependant, and not all games progress at the same rate. This argument doesn't really belong in a discussion regarding tech structure, because it's a call in. This applies to the Command P4 as well, which is probably over performing.
Also, no one builds both T1 and T2 as soviets unless you are playing completely for call ins, which isn't really viable this patch. The tech costs I provided are how most games play out. Sure there are niche tactics that might work if you outplay your opponent, but for the most part soviets go either T1 or T2, then T3 and T4, while ostheer goes T1, T2, and T3.