Shot blockers? Are you kidding me? Get a flame ht and attack ground over the shot blocker. Mortars and grenades still work too. If your problem is with the bofors, shell it till it braces, then attack it in a hit and run. Make sure you know when it will come off of brace, and pull any vehicles and/or retreat before then. Simcities are totally counterable and once you demolish them you've pretty much won.
It sounds like you haven't ever actually played as ostheer against Sim city. It's not viable in 1s due to loss of map control and it's less effective in 3s and 4s but in 2v2s it's very strong. Why? Because the maps are small enough that you can't ignore them, and you have an ally that can cap for you and defend you when under pressure.
Early game:
Mortars die to Brit mortars or can't get in close enough
Flamer HT gets hard countered by Bofors or any AT platform
Paks get decrewed by mortars or forced out of range/sight
Pgrens with Shreks or flamer pios get countered by Bofors
Mid game:
All t3 tanks get halted by single AT since they can't flank due to high penentration of bofors.
Late game:
Ostheer needs werfers or they'll have an extremely hard time.
Note that this is only the case if ostheer doesn't pick the right commanders.
I don't typically struggle against Sim city but if I make one mistake trying to counter it it's an uphill battle from then on.
TLDR: ostheer counter to Sim city doesn't come until its too late.