I’m not sure I see a reason for the stepper to lose torque, generally steppers and drivers work until they don’t work any more.
It could be that friction is building up somewhere and using up stepper torque?
I did inadvertently test holding torque whilst testing deflection and stretch, but this is really not the sort of thing that the Carbide support folks are going to recommend that you do to your machine…
This has been done before, answer was about 18lbs force I believe;
The other way to do it would be to hold the Z carriage back with the luggage scale and step away from it, still not particularly good for the motor or belts though.