Although I agree with you, reducing the over dominance of Arty and TDs shouldn't goes through directly buffing mediums but probably nerfing Arty and some other stuff going again medium tanks play. Personally I don't think TDs need to be nerfed if we can make medium tanks more reliable on the late game (using various upgrades)
If you nerf artillery, you make team weapons stronger. That makes medium tanks weaker, not stronger.
The simple fact of the matter is that 4 hit mediums just aren't ideal for laney maps that favour team weapons. There's just too much anti-tank firepower around to reliably preserve them.
As a result, more survivable tanks (long range tank destroyers, heavy tanks) are favoured on that sort of map.
That's not inherent to teamgames but to the map pool. If you made a 1v1 map half the width of Kharkov, it'd turn into an artillery showdown. Likewise, if you played a 2v2 on a 4v4 map, more mobile, versatile units like medium tanks and infantry would be more powerful than more static, higher firepower specialists.