A very simple idea: if you manage to destroy all of the enemies' shipyards, and prevent them from building new shipyards you've beaten the enemy and can demand anything you want. A small fighter in outer space can completely dominate a giant class 26 world if it has local space control because on the turn the class 26 world builds a shipyard, that fighter can shoot it down. And if the world doesn't happen to have ships on it, THAT WORLD CAN'T DO ANYTHING ABOUT IT UNLESS REINFORCEMENTS COME ELSEWHERE.
In other words, you don't EVEN NEED TO INVADE THEIR WORLDS. IF YOU HAVE TOTAL SPACE CONTROL YOU'VE WON. Also, I might point out that there should be a planetary bombardment option where the goal ISN'T to invade a planet. Instead it's to bash it to rubble.
THIS IS SOMETHING YOU CAN ALSO PROGRAM THE AI TO TRY AND DO, AND TO RECOGNIZE.