Great stuff Marc. I think you hit it on the head when talking about how the mechanized infantry sculpts show a broad definition of mobile infantry, from troops on trucks (the hobilars of their day, with the vehicle being just a conveyance) to tracked fighting vehicles with highly trained infantry contained within.
I like your split into three units, but I don’t know how, using the existing combat system, to create a mechanic for tank destroyers to target certain units. The system is a little too abstracted to make it work, methinks. But it sure would be fun to try.
However, the mobile AAA would be great, and combined with self propelled artillery would make a perfect combined force - armor, artillery, infantry, and air defense, all able to move as one. That would be fun! My only concern is that historically these sorts of devices numbered in the hundreds, whereas a tracked self propelled artillery piece like the StuG III pushed 10,000 produced. So it’s a minor scale problem, but hey, if someone wants to buy a fleet of shiny new mobile AAA units at 6 IPCs a pop, more power to them :-D I know I’d buy at least one - couldn’t help myself.