Welcome to the forums! Glad you like the game.
As far as difficulty, it definitely gets harder. As you progress in the game and take more planets, you will notice the "AI Progress" go up. The higher this gets, the more the AI is going to retaliate.
That said, level 7 is considered "normal" difficulty, so you may never find 6 all that tough. But, if that's the case, fear not, things get tough real quick after level 7.