Alright, then would you feel better if SU-76s came later but everything stayed the same? That seems to be the message you're giving me. I wouldn't mind if T3 was pushed back by 10 fuel or something since M5 and SU-76 are problem units, but there is really no elegant way to push SU-76's back too much without crippling SU. Nerfing them would spell disaster as they would be locked out of a reliable TD until they have enough fuel to afford their most expensive tank in the last tier. I feel like the design of SU in CoH 2 doesn't leave a lot to work with. For now, a scaling TD that arrives mid-game might just have to be a thing that Axis have to work around.
But timing for the SU-76 is a little off-topic, so I will leave it at that.
I think in the last patch , Relic has broken the fundamental rule of balance - if you're making big changes, go big. They've changed a lot of the roles of units when it comes down to the narrative of the game and timing, but forgot to change a lot of the stats of the stuff they've changed, so it leads to the current M5/SU-76 situation we're in.
I feel the easiest thing to do is to make the SU-76 a purely early-midgame unit. That is, a unit that can deal with P4s and nothing heavier than that, with barrage being a bit more accurate too (so give SU-76 ZIS stats for barrage). People are still going to build SU-76, and it is still strong early to mid game, but leave it to that really. (IE a Pennetration nerf, a range nerf would hurt the SU-76 more than a penn nerf)
In most strategy games you find, if a unit can be spammed, its because its really good at one stage of the game, but its really limited in others/has reliable counters. If the latter doesn't happen, then we call that unit broken. The most famous example is, of course, zerglings.
I have no problem with T-34-76 spam in the past, I had no problem with M10/M18 spam in CoH1 but yeah, I think talking about the SU-76 in this discussion is kinda derailing your point a bit because it is a controversial vehicle, but I mostly agree with all your points.