Today I just beat the whole game! Tutorials, weapon training, races, challenges, you name it, it's done! Every single button for accessing the designer (like the sandbox's "Build and Fly" button) has a green light next to it, meaning that it has been completed. This isn't big news for you people, but for me it's a huge accomplishment.
For those wondering, the most difficult part was probably the "Rocket Training" part in the Combat section, I had to use a plane which had more than 200 rockets to finish it.
Also the fact that the "hardest" challenge (Daredevil), I actually completed it FIRST TRY!
Anyways, if you have any questions, tell me in the comments!