Stepper drivers are current drivers, they don't step down to the motor voltage, because the motor is inductive, they convert to a current. Higher supply voltage on the driver allows faster motor rotation by overcoming back-EMF.
Stepper motors have a current, resistance and inductance rating - voltage is not really relevant as inductance and back-EMF usually dominate its behaviour.
BTW it is far easier to simply piggy-back an existing DRV8825 module onto your board - then you can easily replace it if it blows up too.