No God did not choose his nature, but why would that make him a slave? In what sense? By not having absolute freewill? (is that even coherent?) If so, that's a concept of freewill that I don't find obviously true, and thus, you are going to have to defend it. It's a logically impossible task that you are asking God to do because God cannot not be the way He is by definition; in other words, it's logically impossible for God to choose his nature or be different. And no that wouldn't compromise God's omnipotence because omnipotence doesn't include the ability to do logically impossible things, and that wouldn't compromise freewill because the task can't possibly be chosen. I don't know why anyone would subscribe to a version of freewill that advocates choosing something that is not choosable or being something that can't be anything else. By your conception of freewill, I wouldn't have freewill because I didn't choose to be human or a beaver or I wouldn't have freewill because I can't teleport, create universes with my mind, nor make angels, or choosing to exist and not exist at the same time.