Really it's not rocket science as to why Soviets slightly overperform at the moment. The 7-man upgrade is too powerful currently for its cost, this doesn't necessarily mean the upgrade comes too early as Cons still aren't amazing early game but they are probably the most cost-efficient squad in the entire game once they are bolstered and vetted up. At only 17 mp reinforce cost they are roided up versions of Ostruppen. I would be for the upgrade itself receiving some nerfs, maybe an increase in cost and possibly some slight nerfs to performance in cover to make them a little less efficient against certain units. In change you could make the upgrade not require molo's and AT Nades and be up-gradable once you build Tier 3.
Before 7-th man was implemented, i suggested make linear upgrade (with medics you got decreased cost of reinforce, with T2 time of reinforce decreased) and replace flare mine by passive fast sandbag building, but without any combat-bonuses. With T3 got 10MP cost reduction to new squad and with T4 another 20 Mp. Than you could get "mass, cheap" infantry. They don't have any in-combat boosts (besides fast sandbag), but very cheap in maintence. Idea was make cons scaling with natural teching like OST teching works. IT's much easier to balance to prevent huge powerspikes in certain timeframes.
The T-70 is in an odd state because it does come late but it is the strongest light tank in the game against infantry and can swing the entire game almost immediately if counters aren't present.
If you don't have any counter to flame-ht you will be screwed as well. Any enemy move need counter-move. You need counter to M5 Quad, that could lock big zone from you; need proper counter to ass-gren spam or mg-spam.
I'm not sure if the problem is the T-70 being so strong against infantry that you almost have to go for a Puma to counter or if the Raketen is just too squishy against the T-70. I feel the T-70 could probably be made cheaper but have its performance reduced slightly to compensate making it more like the Panzer II so the SU-76 can fill a Puma-like role in Soviet Tier 3. Plus if T-70 gets toned down we could see more M5's which would be nice.
SU-76 NEVER could be like puma (don't have turret, smoke and much worse mobility). Puma could dive any LV, but diving su-76? I suggested make from su-76 arty support unit (could be deployed as kv-2 and use regular barrage for free and smoke barrage), while have penetration potent only against LV. In that case you could have AI tool, but with indirect style, while T-70 is more aggressive style.
M5 good (much cheaper and good suppresion), but only 2 shots from pak or rak and it's gone. T-70 could forget you some mistakes. M5 don't.
Possible decision for t-70 - switchable rounds, like it was in reality.
1. Shrapnel shots - lower rof and pen, but have aoe. Tweak damage and range. You still could deal damage to infantry, but wih lesser range, that make it more "micro-oriented" risky unit.
2. Ap shots - higher rof and pen, but with much less AI power
Heavies in general need to be changed but the IS-2 just does everything well. With reduced scatter it no longer struggles against infantry and can wipe units rapidly with decent RNG, it also has really strong armor and can come pretty early at 10 CP. Which can often be a disaster for a player who opted for a medium tank before going for armor that can deal with the IS-2. In general heavies probably need to be delayed somehow, maybe an increase in cost, CP's, or further tech requirements for them. At least for the Pershing, IS-2, and possibly the Ostheer Tiger.
Heavy meta ALWAYS was win or lose decision, you trying to survive against enemy mediums push while you don't got your heavy. It's very close to T-70 situation, when you have it, you could push enemy, but when first medium arrived, you are 70 fuel down and need wait until your t-34 arrive. THe same with heavies, you could push by mediums your enemy while he waiting heavy.