My experience with Gran Turismo was miniscule before 2023 and GT7. The Lightweight K-Cup was the first class of races that baffled and frustrated me on how I could not even crack the top 5 on most occasions despite trying all the different Kei cars and upgrade combinations to stay under 400pp. Now I have a go-to Kei car that crushes the competition on hard difficulty. It is the Honda Beat '91. The catch is that it requires the K20C1-Civic-'20 engine swap. You must be level 50 to buy the engine or it is possible to obtain it by chance through the roulette wheel.
You can use all the same settings from the first 2 columns of settings on Praino's
Honda Beat '91 - 537PP - Sport Soft - All Track - Swap - 1.39 tuning but use Comfort Soft tires. I also used all the same upgrade parts/settings he has listed for Supercharger, Intake/Exhaust, Brakes, and Drivetrain. However, for non-removable upgrades, you only add Bore Up, Stroke Up, Polish Parts, weight reduction 1-3, and Increase Body Rigidity. Since the <400pp version doesn't have as much Max Power as the 537pp version, I also reduced each gear in the gearbox by 10km/h. I also did not get the wide body. It is so overpowered you need to be careful when turning and braking. But once you navigate through a turn, it can catch and pass any vehicle that K Cup will throw at you.
I used to despise any K-Cup race, but now I look forward to them.