I personally would love to have SU-76 as OKW. Imo the units is cost effective there as simply better option for the Soviets.
The reason I mentioned the Elephant is simply because some people seem to think it can only deal with light vehicles when that is not the case when in numbers. Another advantage when in numbers is that if they are kept apart the can hit the read armor of units trying to dive.
It would need a major rebalance though in terms of pen chance if you switch sides completely. After that I'd probably still take the Puma if I had the choice as OKW.
But I also don't think that the SU76 is a viable late game choice. 3 of them can dish out decent damage, but they need to be spaced optimally and as casemates they need a lot of support and micro to be able to deter heavy armor. They pen Panthers with slightly above 60% chance and Tiger with sligthly less than 55% chance at long ranges. And they need 9 pens to kill. The chance that your SU76 with their low health pool get chipped away one after the other during multiple fights or you lose one to a flank is high. You will only bleed more and probably don't do too much. Every faction will be better off with a real TD in the late game.