The problem there is unless you start getting to insane armour values, all you achieve is cementing tank destroyers as the only counter.
I don't know if this is even within the capabilities of the mod tools, but what if instead of more frontal armour, they had frontal damage reduction?
Say, they've got 720 HP like a premium medium, but 50% DR on the front, meaning attacking one head on gives it 1440 effective hit points (the same as a Churchill?).
That'd make them more effective at the spearheads they're designed for, but also much more rewarding to flank.
Absolutely! That’s actually a great idea!
That way they are still not able to push through your entire army up front, but still effective leading the charge, and by flanking with TDs or mediums you can hurt them badly!