Your flawed thinking in that the su76 will hold off the enemy mediums all by itself, it won't. You need support. And it will do well in that regard. It's arguably better designed than the StuG because of that. It's part of your army not the whole damn thing... If you want a single 70 fuel TD to stop the enemy armour in its tracks you are sorely out of touch.
??? No my part army and support will be like similar to this:
With T3 1-st unit will be T-70 (approx. 9-10 minute of game) it will give me good field pressure and strong and potent AI, it also could fight against LV. 1 Zis gun to counter enemy medium. Than T4. I spend only 70 fuel on 1 unit in T3, have good field pressure and enough AT, before my T4 will be deployed. 1-st P4 will arrive approx. at 13-14 minute. In this time i almost have enough fuel on T4.
Then please told me how looks strat with SU-76???
T3 deployed. 1-st unit t-70 (i will get field pressure, AT against LV and AI), 2-nd unit su-76 (i just delay my T4 on 75 fuel and spend MP that i could spend on zis-gun and backteching to zis and it will be approx. 13 minute). P4 arrive with my SU-76 or slightly later. Than you have: t-70 and su-76 against P4. You also have or penals with PTRS or cons with at-nades and T2. Who will be more aggressive OST player with P4 or you with t-70 and su-76 - open question. After 5-6 minutes will arrive 2-nd p4. In this time you could build T4, but not any T4 unit.
T3 deployed. 1-st unit SU-76. I don't have field pressure, i have barrage against static positions and hardcounter to any LV. German infantry have weapon upgrades. I can't deal with them by su-76 and have more bigger MP drain. In 13 minute will arrive P4, i have enough fuel to T4. I could have cons with at-nades and zis or penals with ptrs. BUT my cons will be suck against german infantry with weapon upgrades. I could make 007 con upgrdae for 25 fuel and 100MP and delay my T4.
As you see in any possible way you don't need su-76 AT ALL. I don't look on situation when you have T4 and build su-76 instead trying to save fuel on su-85.