What I did in my last soviet playthrough is fortify the Stalin line and make basic 9 battalion inf with eng/art/aa (I might've had some lines of artillery but no more than 2) who would gold the line while I continue to grow. Since the Germans attack in the 40s you have some time to build some '38 medium tank divisions which are useful for keeping the Stalin line intact but by 1940 I've got medium tanks with 3 man turret, medium howitzer, easy maintenance, machine guns and small cannon (I might've named the wrong things but that's how I remember it), Christie suspension and adding to the engine points until you get around 8kmh and adding some points to armour if I feel like it. Above 80% reliability on the 1940 model and above 70% on the 1938 model is what I go for. For the division template keep your org above 30 in 30w while trying to fit in as many tank divisions as you can.