I think a more significant problems is that either medium tanks are too expensive, or heavies and superheavies are too cheap. Due to the fewer tanks required, a heavy tank battalion is only slightly more expensive than a medium tank battalion, while a superheavy battalion is actually cheaper than a medium one. Because of this, medium tanks aren't very useful, regardless of stats. One particularly egregious example is later SPGs, where a medium SPG battalion with heavy howitzers is actually both weaker and more expensive than the heavy version, though it doesn't require as much supply.
Division-level issues aside, I don't really think there's a place for an advanced medium gun. The second medium gun is a very effective weapon, since it provides balanced SA/HA without breaking the bank. Rather than the lack of an advanced medium gun, I find the early guns to be completely out of whack. The most egregious example is the close-support gun vs the first medium howitzer. The close-support gun has equivalent SA, is cheaper to produce, and does not require any resources, while the howitzer's only improvements are to HA and piercing IIRC, with neither of those stats even being meaningful for it.