Here’s the problem. Below is the theoretical torque speed curve for a 393 motor, notice that stall current is around 4.8A. Now to protect itself the motor has a resettable fuse known as a PTC which will after a few seconds disable the motor, but for at least a short while it will pull lots of current. At maximum motor power, the motor is still pulling over 2A, this is all at battery voltage of the cortex which was 7.2V.
The 3wire device cannot provide 7.2V, only 5V. It also is limited to 2A maximum for all 8 ports combined. You can see that’s it’s not really intended to drive motors. So you could use a 3wire expander, but it will also be limited to 2A.
It sounds like a redesign is needed (you really would not need 6 x 11W V5 motors) or perhaps you could substitute the 393 motors using the smaller 5.5W V5 motors rather than the 11W.
There may be some around from years ago, but you never directly send voltage to these motors, rather, a pwm waveform is generated in the motor controller (MC29) that is proportional to the command speed you send to it. The actual motor speed vs command is quite non-linear for an unloaded motor. You will find some curves like the one in this old post around though.