So what exactly should be the stat choices for non-vehicle variants?
For artillery I was thinking:
Reliability, this seems to be a standard. In this particular case it would represent spare parts, carts, heavier rounds and propellant charges being taken account by supply system, beyond just sturdy equipment. Logistic investment per artillery piece essentially.
Bore*, increase should increase soft attack, breakthrough, some hard attack and defense, and maybe a bit of penetration. The offensive firepower increase.
Power**, representing increase of muzzle velocity. Increase should increase soft attack, penetration, defense, some breakthrough and hard attack. Defensive firepower increase.
Lightening, represents adoption of lighter carriages made possible due to more powerful recoil systems, alloy metals, and simply accepting higher stresses. Both the firepower increases should affect terrain modifiers since they result in heavier gun, this stat would allow you to counter those and also represent pack artillery if you focus on this.
With those it should be possible to represent average artillery park of majors and we wouldn't need to wonder what kind of artillery exactly is included under ingame "artillery". Japanese would focus on cheap and portable, Americans and Germans on bore, Russians would field lots of guns (as opposed to howitzers), while British would go for balance.
*/** A tentative suggestion, feel free to argue how to better represent these.