Well, as noted, it's only 30% of planets in general -- with defensive AI types, it's a lot more of them. If someone likes to go a-steamrolling, they can always play against mad bombers or similar, by the same token.
I support choices also, and I don't think that having 30% of the planets be blob-immune is a reduction of choice by as severe a degree as you make out.
By the same token, increasing the amount of ships it takes before going into effect pretty much makes it pointless: if you have more than a 2:1 ship ratio against the AI, boy do you overwhelm them. Especially since you can choose to engage their guard posts peacemeal, that means you'd often outnumber them 10:1 or even more in each sub-battle.
On the other hand, if you want to just steamroll the AI and you have THAT many ships, there's one always-around standby: just send in a raiding party to cap their command station early, then go nuts against them with the full battle that ensues (since the AI Eye would die when you cap the command station, whereas it does not at present).