How to dynamically regulate voltage from a DC power supply to motors
A simple way is to use a voltage regulator and if your motor current is 1.5 amps max then I would recommend using a buck voltage regulator like this: -



You can vary the output voltage with the resistor potential divider that feeds the FB pin. Power efficiency remains about 90% for a good range of loads and you will never get anything like this from a potential divider and, a potential divider would never give you a regulated output voltage.

If you need more current then this might be attractive to you: -



There is a version of the LT8610 that has an FB pin so you can set the output voltage. You pay for what you get and if you want good performance you need to pay a few dollars more.