If your goal is to have a race with cars of somewhat the same PP and you are not goingt to multiplayer but instead want to drive against AI, there is unfortunatly no other means than asigning each AI driver a car from your garage. The other method would always lead to race cars instead.
But even then the AI on professional wont be any better than during all of the premade existing events.
It is even worse, if there is a slower car sometimes the AI is hesitant to just pass and keeps following for long durations.
2 Solutions to this:
1. you need to give the AI cars that will overpower you by a bit, maybe give them racing tyres while you stick on sport or whatever other method may seem to produce better results to your liking
2. the fastest AI cars need to be put in front to reduce the chance they will group up behind a slower car. Once they start lapping the slow cars will yield to the faster one. Also this puts more pressure on you to follow up as a sideeffect.
But this is the most bandaid you can get out of AI racing.
Different approach is to challenge yourself with any sort of personal set goal like "be faster than x total time" within circumstances.