I'm surprised no-one's mentioned some of the obvious reasons the OP might have found a huge jump from 6 to 7. You mentioned One-Way Doormaster, but you haven't said what the other AIs you fought were. AI types make a huge difference to difficulty. Also, randomness plays a significant role. For example, some bonus fleet ships, such as Zenith Bombards and Eye Bots, make terrifying unlocks for the AI. Others, not so much.