T70/SU76 still suffer from being too high risk too low reward.
I agree with most of your observations, especially this one. I still believe that the SU76 and T70 should be swapped between their respective buildings. With the increased fuel costs for both vehicles, any minor improvements these units received are lost due to them still coming far too late and essentially being a weaker version of the unit's they share a building with.
I'll restate here why they will find more use in different buildings:
T70 is a weaker version of the T34/76 and in most cases a player is better off simply waiting to build the new and improved T34/76. Same applies with the SU76, it is a weaker version of the SU85 (albeit the barrage is a decent ability), and most players will simply wait for the SU85 while stalling with their ZiS.
If Relic were to swap these unit positions between buildings, you then have the SU76 providing reasonable AT from range while the T34/76 engages, while the ability to barrage gives T3 a form of light artillery it is currently lacking; you would also find great use for the T70 in T4 in the way of being able scout at range and provide sight for the SU85, while also being able to provide close range AI support to the SU85.
I know it's unlikely that Relic will adopt such a change, but the fact remains that despite their currently stronger late game potential, the Soviets remain a generally boring faction to play. It seems, currently, that the lack of combined arms in the Soviet mid to late game is only less noticeable due to a few over nerfs on the Germans. We are close to pretty good balance again, but it doesn't make the Soviets a compelling faction to play, and until the Soviets get the ability to field combined arms, Soviet late game will always suffer in any instance where the units between the factions are properly balanced due to a lack of combined arms options.