the ability to instantly buy a supercar is great, but for a sense of progression you can make it so that you have to win races to be able to afford better cars. S1 class cars should be more challenging to get, the class rating should reflect the difficulty of obtaining it. Having it so that you work your way up to faster cars, not only gives the player the sense of progression, but also teaches them to drive better. By starting with a slower car you can have an easier time understanding how the car moves. As you drive the car you gain credits, passively, slowly. You also get credits for winning races. you can focus on one car, working on it, making it better, upgrading it, making it special. It gives value to having a collection of cars, its like not only do you just have a vast amount of cars that you never drive, but each and every one of them you bought and used and leveled up yourself. It gives that completionist feeling.
upgrades like roll bars can give you access to a portion of the tuning slider, more specialized upgrades like that can be available, fluffing up the shop with options that are actually meaningful. On top of that, the people who dont tune, will get the chance to see what the different upgrades are like. You can emphasize the fact that these roll bars are the stiff ones, they give you access to this side of the slider. Then the more expensive ones give you the whole range. same goes for the springs.
maybe also have a completionist thing, where purchasing everything for a car gives it something special. i dont know what that would be since cars are fully customizable, but maybe like a car level or somtething.