If you hesitated between two or more cars in a career series, it’s worth going back and playing them again with a different car. And because Builder’s Cup events are not time-limited, you can actually take the time to do practice without having to feel the pressure of the ticking clock.
Otherwise, if you want to play against AI, you have Free Play. The main difference between Free Play and career is that you don’t get grid positioning (unless you are willing to fiddle with group settings), you don’t get practice, and it can be quite annoying setting up some restrictions or group races.
Then, if you don’t mind driving on your own, you have Rivals. It’s a good way to just test out some cars, or test out some tracks. You can even measure yourself against other players for fun and profit.