The design intention to keep the super cheap units viable vs the big leaguers, coupled with the ridiculous accuracy system, just breaks things. I agree with you that the T34 and 55 are too much, its just that the Marder pretty much embodies this issue as a whole.
Its something that needs to be nipped in the bud ASAP, because its gotten quite ridiculous. This calls for a big Tigga-esque topic with suggestions
Well I can compile all the suggestions I feel are good:
1) Experiement with lowering the 5% cap on accuracy on "main gun" type weapons. Potentially to 0%. This wouldn't affect big tanks/vehicles that fire few shots nearly as much as small tanks/vehicles that fire many shots. You have to be careful not to overly nerf machine guns or autocannons like this as the same change applied to them might skew things a lot.
2) Morale penalty for people dying around you. The problem is that this would probably have to come with a morale boost overall... though I'm not sure people would mind that. Would be tricky to balance.
3) Increase chance of criticial when AP is much larger than armour. One of my issues with the VTS1 is that it can live through most pact ATGM hits with 1 hp left. Massively better than the Zhalo which is killed by any ATGM hit. This would make large AP units much more viable against low armour targets.
4) Limit availablity of prototype units a bit more. This is a little more general, but would hit the VTS1, the Zhalo and a few other units that are a bit unique and can cause complete carnage in the right situation. The MBT-70 and the BMP 685 for example. I think it'd mainly hit the Zhalo and the VTS1 though.
5) (Marder series only) Increase the price of the Marder 1 and the VTS1 by 5. Don't really mind the Marder 1Ax right now, but the VTS1 and 1 are both underpriced for their armour. Alternatively for the VTS1 you could hit it with some reloading while moving nerf due to the external cannon.
6) Make reversing a bit easier in the interface. Ctrl-click, or increase the distance behind I can click without the unit turning.
I think a combination of 1) and 3) are fairly easy to implement and both would shift the balance more toward higher AP, higher armour units being effective. Lighter units would still have a place, but that place would be either much closer up where armour is less important, or sitting on the flanks sniping away at heavier units' sides and rears while having the speed to retreat if things get hot. Lighter units would keep their ability to push back heavier units through weight of numbers, they just would just now have to pick between inflicting no damage on on the move while taking hits and stopping to take shots, slowing their advance.
EDIT: Just for clarification I'm not sure I'd want either 1) or 3) to be implemented too strongly. Maybe drop it to 2-3% for 1), and only increase damage with strong AP vs armour a little as well. An alternative to 3) might be to add a little bit of a random element to damage other than criticals. An AP 10 ATGM hitting armour 2 currently always does 9 damage, or rarely 18 (I think). More interesting would be to have it do between 7 and 11, with the same chance of doubling up.