I thought I figured out a way to do it, and apparently building a sub pen early just sets the enemy MCV in motion to the other island early. -_-
So, that didn't work.
I guess maybe build up a ton of stuff first; have troops ready to go in before building their transports, and don't destroy their upper base until ready to deal with their second base right away. Once they get to that stupid island, they have infinite money basically, so infinite troops to yeet at you. -_-
I might bother trying again later, but for now this just frustrates me too much and I can't stand it. Well, here's how to make me hate a mission though: design them like this, and it makes me want to never play it again. -_-