I'd say you're overrating shocks, and underrating guards and mark target. No matter the game size I'd rate guard motor and guard mechanized the top two commanders for soviets period. Third best is Shock rifle for 1vs1 and counterattack for team games. Shock motor acts as a gimpy, free version of guard mechanized. It's better to pick it than anything without ISU, but the guard one is simply better so there's little reason to ever pick shock motor.
Mechanized support doctrine (Guards/Mark Vehicle/ISU) has been my default doctrine for a long time, but since WFA release I can't get Guards to work in 2vs2 games as they got raped by Obers/Jaeger/Falls or Gren/LMG blobs.
Now I'm stuck with Shock Motor (Shocks/ISU) or Shock Rifle (Shocks/IS2), just because they have shocks that can hold the ground against OKW elite infantry, but I really miss Mark Vehicle because without it my T34/76s feel useless against heavy tanks and I have to rely absolutely in call-ins tanks.
Are currently Guards really competitive in 2vs2 against decent skilled axis teams? (Usually I move between rank 150~500 in the ladder)
How many Guards do you usually build? Do you support them with T1 or T2?