During the regular round of combat do they hit on 2 or 3? (2 would be my preference).
So, if I attack with 3 against 1, then its automatic superiority. But OOB rules its 5:1 (I need the math confirmed). So, its a trade off between a chance of prevailing on defense vs. surviving in normal combat and giving up superiority. (Maybe the defender should have choice, multi vs single round?)
Also it would seem the sky needs to be clear of enemy aircraft to provide artillery support. So since the premise of your rule to provide incentive to purchase aircraft (which I agree), then allow the defender (after the 1 round of air superiority) to retreat them. Then the attacker would have clear skies for artillery support, or the defender can choose to stand and fight. The defender can only retreat to a friendly non-contested territory (must have 1 inf unit of same power). This rule would also just be hit on 2, and retreating defending aircraft do not get to roll.