ahh, forgot that the truck with flak can convert resources, but still flak can be countered by guards, obviously not in open field but in building or green cover for sure, M3's are providing a great counter to sturmpios and they are expensive to build and then to reinforce.
I'm still thinking how OKW can survive playing vs soviet windustry doctrine
I'm not saying that the balance is perfect but I think Mechanized Regimental Company offers the best options for dealing with this.
I am going to assume that before CP2 you get the tools you need to kill the M3A1. You can get 251/17 up very quickly with resource conversion.
I am also going to assume that it gets vet 1 because it is cheap and does considerable damage. 251's Vet 1 power is a Smoke Screen power that can protect it from Guards buttoning fire. 251 has greater range than Guards. It has far greater range than it has sight. With the right circumstances and with units screening and scouting for the 251/17 it can act as a good fire support weapon and still battle even when Guards are present.
The Puma also gets smoke immediately for OKW so you can escape buttoning fire from them as well. Puma wrecks the M3A1 so Guards cannot use it to chase. It's a fairly potent defense and the Puma scales up against the T-34 pretty well. You don't want to fight it 1v1 but it can easily turn a fight with support.
If you survive until CP 2-3 you now have better options for dealing with Guards as well. You've sunk fuel into your anti-tank and suppression weapons so you can sink manpower into Fallscrimjagers. Fallscimjagers can kill snipers from behind pretty well and I bet they do pretty good against Guards too. I find that Fallscrimjagers are a little inconsistent and much prefer Obersoldaten but I think the early fuel investment will push you away from them. The MG-34 would also help deal with Guards as their immobility makes them a weak flanker.
You might be able to make other doctrinal infantry work as well. I've heard good praise for G43 Panzerfusiliers but if you're converting out of munitions and probably using smoke rounds so I don't think Breakthrough Tactics will offer much aside from the Sturm Offizer.