Dayum. I'm impressed. I enjoyed the game a lot but still I gave up on getting all the stars, long before the end.
Game is not balanced on Expert. Some races are extremely easy but others are hard as hell. But i enjoyed every single minute playing it, even though raged few times

Or are you?
That's a good start, but don't forget time-trials!
EDIT: it's where mastery of the game is required. I managed to get a S-ranking for the whole campaign without even looking into risk boosts...
I did few Time-Trials on Expert and i got all gold medals in Time-Trials but as i said before game is not balanced on Expert and i hate that XD