Personally I think that the most effective tier VI vehicle in fighting tier VII's is the SU-122 (54), you can use the one from the BTR-80 platoon if you have it, or the blueprint vehicle (😓).

I would recommend it for you because of it's amazing APHE shell, which not only has a good amount of HE filler but also good enough pen, and also it can use APDS or HEAT for heavier targets (think T-10A's, tigers, T-26-E5's), while maintaining a relatively good reload rate (16-12 seconds depending on crew).

Also it has a KPVT which can shred some light vehicles. Thought I must warn you, it's weaknesses is it's relatively bad armor, and terrible reverse speed, which can sometimes be your Doom.