You don't have to though, just bring your best car that fits the regulations and have fun. If someone else in the race has spent 8 hours fettling their car and beats you, it's not the end of the world is it? Eventually I'm sure you'll have a solid range of cars suitable for the different parameters the game gives. A 500PP hot hatch, a 750PP Hypercar, a 400PP RWD Japanese car etc etc.
I get that you want a level playing field but as I say, the past GT games have never really been about that. The game wants you to upgrade your cars in hundreds of different configurations with all the parts available, which GTS didn't have. Trying to apply BOP would be impossible, so they're relying on PP.
I'm sure at some point there will be fixed events for race cars that can't be modified though, or stock road cars.