I fiddled with it further and confirmed that
both "Steel Armor Front Max" and "Armor Engagement Range" should be set to 255, for the Stugs to engage the enemy armor.
But then, they wouldn't engage the trucks. So I have this OpFire Filter setting applied on them:
Good thing there's that
Save and
Load profile buttons (blue and red, respectively) so you can save such settings, and quickly load them to be applied on each and every armored platoon.