@Epicredeemer
Honestly, you probably wouldn't even need something on the scale of a modern tank cannon to penetrate 2cm of, at best, high medieval grade iron or steel. I doubt the empire has the metallurgy for anything equivalent to face hardened steel used even in WW1.
Even assuming that their metalwork is as good as modern stuff, we'd really only need something on the scale of a 37mm cannon from a PzIII, or a modern day 40mm Bofors to reliably get through at about a kilometer. However, like you said, just because you get through the armor, that doesn't stop the threat unless you hit something vital. Which is why I like the HESH idea for lots of spalling.
This is kinda trouble for the JSDF because most of their heavy support was in the form of AH-1s or F-4s, and their cannons wouldn't be able to deal with the plate, much less a .50 BMG, although I imagine rockets and bombs would probably be a little more effective. On the ground this means the JSDF does need heavy mechanized vehicles, because nothing in their inventory to my knowledge can deal with the behemoths beside their tanks, and normal foot infantry without AT gear wouldn't be able to deal with these new threats.
In a way, the empire has hit a sweet spot of armor protection. Nothing they could ever make will stop a 105mm cannon, but instead are going with can stop the majority of the JSDF arsenal is plenty good as a countermeasure.