I have a quality-of-life suggestion: When you turn a unit to level 3 you get a bonus unit, but if you don't use that unit and turn another one to level 3, you end up not getting an additional unit. Maybe some sort of "layering" can be added where the unit is added "behind" the current one, maybe they can even be switchable, or you could even make the unit change to one of an even higher level... just throwing ideas.