Controlling speed of 7 DC motors from Arduino 101 using PWM outside of the audible range
Reason for PCA9685 quite low frequency is its main use is led driver. I recommend you to pick motor controller IC which is meant to do the job. Also I wonder how you are talking about 12V supply because I checked the datasheet and it pointed out absolute maximum input 6V (see the attached picture).

Based on fast check I2C interface is quite uncommon in motor controllers, at least in multi output ones. Seems like its easier to find drivers with SPI interface.

I'm not sure about premade modules offering such functionality (Excluding expensive industrial controllers). Maybe you can find some hobbyist drivers tho.

How about implementing something like this:
http://www.ti.com/lit/ds/symlink/tpic2050.pdf