Self Contained Movie with Current Settings is as good as you can do. That's a straight sideways export neither gaining nor losing data.
Most things you do to "help out" will add distortion by making Final Cut "convert" to a different format.
The only way to increase the quality is start with higher quallity clips.
There is a way to convert to uncompressed at the beginning of a project to make special effects and other processes far less damaging, but that's the only time converting after the fact adds anything to the show, and that's also assuming you didn't start with top qualilty at the beginning.
Koz