Another possibility for HBs which SBR is that they only hit on 1-4 which means they max at 8. I’m sure not all bombs hit their targets. Also consider capping the total damage at 1-1.5 the value of the territory.
Just roll them all at once. As long as you use different colors for different attack powers (ie all 1’s roll white, all 2’s roll red, all 3’s roll blue and all 4’s roll yellow or something) or you just roll for each attack power separately, you are fine. No need to roll each individual bomber since all the hits total up at the end anyway.