Another problem I found with driver boards is they have amplifiers on the chip for "step"
that picks up ac hum like touching an input on a regulator sound amp making annoying buzz sound.
When operating a driver with a stepper motor ,will buzz too.
I found soldering 2-470k resistors from "step" and "dir" to ground will shunt buzz problem without interfering with operation and reversing directions with no motor hesitation.
Now the hard part.TO do the "enable trick" may involve another Arduino uno,nano or ATiny85 to be a slave to the Arduino uno cnc shield to co ordinate enable trick and 555 osc. to your main operation will require some programming skills.
I haven't got that far but will try doing so and publish my findngs.