I've tried a normally energized relay to short the output but the relay doesn't release until the motors stops on its own. Apparently the motor hash is enough voltage to hold the relay.

The circuit diagram has no brake at power off. Note the Brake NFET next to the motor.

Possibilities?

Can we make the relay drop? That, and a capacitor could give a surge of voltage at the brake gate just long enough to stop the motor.

A battery is not allowed in this application.

beyond a relay, Is there a solid state answer?

All ideas and circuit critiques are welcome

 

If your main power switch is beefy enough (slightly more expensive) you put motor on switched pin. PWM feed to "on" pin, and ground to "off" pin.
Motor line in is broken at switch deactivation and ground is applied to both power leads to motor.
If you need even faster braking then you may have to apply a negative voltage.