I think if you want a serious challenge throughout the beginning, middle, and end parts of a game you should try playing against humans. While the AI is generally very poor overall against a human who has some idea of what they're doing, the AI is also extremely poor in the late-game, regardless of difficulty level.
Even if you could add a few more ships owned by an AI you could blow up, there wouldn't be much point.
Seriously, try ICO (Ironclad Online, the Sins multiplayer server). Playing against human opponents who are actually capable of making strategic decisions adds a whole new dimension to the game!