@jddt With M84 Sxx you set the timeout when motors are supposed to go into idle current state (not ending it). You set the idle current as a percentage with M906 Ixx. Motors will never turn them off completely without user interaction. This interaction might be M84 which without any further parameters will do what the title says, it will stop the idle hold mode of the motors and shut them off. The same can be achieved by issuing M18.
First check whether the Enable signal is working properly. When you power up the Duet, the motors should be un-powered and therefore free to move. When you try to home them, or to jog them (with M564 H0 in config.g or sent first to allow movement before homing), they should be powered up and no longer be free to move. If you find they are locked to start with and become free when you try to move them, then change R1 to R0 in your M569 commands for the external drivers.
I use belts to lift the Z axis in my printer. I solved the bed-drop-on-power-off problem by installing a worm gear drive. If the gear ratio is sufficient (and/or the pitch of the teeth is right), the mechanism is irreversible and no brakes are needed to hold the bed.
My printer's bed assembly weighs about 3.5 kg and does not move unless the motor tells it to move.