Actually the name of the achievement it is wrong.
You don't have to "Star up 30 cars" to get it. What you need is "Star up 30 times in total across different cars".
I mean, if you star up with blueprints a car two times, it counts at two. Some cars like the Ferrari "La Ferrari" has 5 stars, so you can Star Up it up to 4 times, and that is the number that counts to this achievement.
I managed to unlock it with only 24 cars on my garage, because I starred up more than once some of them. I don't even unlock any Class S car yet!
So you will get this one earlier than the "Unlock 30 cars" achievement, surely.