Depeding on pitch, a ballscrew will allow the Z-axis to drop if there’s no brake (physical brake, not brake resistor) to hold it in position, and power is removed from the Z-axis motor.
Openbuilds’ “Small” NEMA 23 steppers have 175 oz-in of holding torque. That’s only about 1.236 Nm. You’re going to be able to beat that with nearly every motor in the spreadsheet. Your biggest issue is going to be accuracy/precision. One of those steppers with 16x microstepping is good for 3200 steps / revolution. The standard encoder that people use with ODrive is 2400 steps / revolution, or about 3/4 the precision. Add cogging torque to that, and you’re not likely to be too happy with your part tolerance. IMO you should either change ballscrews to a higher pitch or use gears or a pulley set between the motor and the ballscrew. Or use a more accurate encoder, high gains, and enable anticogging.
Unless you’re just doing wood, then you’re going to be well within tolerance and you can totally ignore me lol