You can incorporate your day trading business and elect S-Corporation status with Form 2553.
For 2021 you want to do this quickly and indicate retroactive to the beginning of the year.
You will have to pay yourself a reasonable compensation (payroll with tax withholding) from the day trader S-Corporation so plan accordingly.
You also can form a separate S-Corp for your used car dealership.
You also will have to pay yourself reasonable compensation (payroll) from that S-Corp as well.
