Another thing to consider is competition.
You'll be "competing" against other rental companies. And they will likely have attempted to have low costs. They rent multiple cars but g et them to teh track in one transporter. They have crew, but the crew services 4 or more cars.
And they have reliable cars that are cheap to maintain.
So they can charge a rate that's profitable to them, and attractive to the renter.
A guy like us though needs to charge enough for our time, (Your family likely wants you home, so there is a "cost" there), and our expenses. My car isn't chosen strictly for it's low cost of maintenance. And we have a righ that takes ONE car to the track.
And, the car needs to be dead reliable, fresh parts, pads, discs, bearings, fuel systems, cooling systems and everything that could possibly go wrong atttneded to. That takes time and money.
Net net, I think it's not impossible for the "Single car renter" to compete, but making serious money is tricky.