I think the concept of the Soviets at the beginning was a somewhat attack to defend type. Although, I guess they are limited in defense compared to other factions. You could always build thousands of conscripts.
You should go right hand with Propaganda and take trenches first instead of bunkers and you'd be fine. One sniper + one PTRD team in trench is good for defence. Plus AT gun and Soviet deadly mortar behind and it's realy hard to counter. Of course you lost with 20 Conscript squads those guys are cap and cannon fodder only! Not to fight.Anyway you have one point that Soviet doesn't have anything to blow the damn bridges, they don't have even mines. But I believe this will be sorted in next 1.10 patch.
Partisans can do that.The IL-2 Can do that.And if your lucky, use the Mortar and the Satchel Charges in combonation and you MIGHT get one good blown up bridge