If I pay 30K for a new car and 50%+ of it for landlord duties, can I claim 15K (50% of the car value) instead of mileage deduction on my taxes next year?

No, you can not claim 50% of the car purchase price as a deduction on next years taxes.  You can only use either of the mileage deduction methods.  In 2020, you can write off 57.5cents for every business mile or use the actual expense method.

[Edited 03/10/2020 | 2:16 pm PDT]