An update of the setup photo.

Now let us agree on how to setup, test, measure to torque constant of the JVL MST342 stepper motor.

Please confirm if I understand your setup/wiring correctly.

@Amirmkr It is not clear what your PWM = 255 means. I guess you are using Adruino/Teensy4 PWM 255 on top of the AccelStep which would make the control signal a bit completed.

I usually test by hand, manually, use PWM signal from a PWM/square wave sign gen (See my chat records for more details) So I would put away the MPU Arduino/Rpi, and use 8kHz (ie, 8000 steps per second) to the step input of driver, and dc 0V/5V direction signal, ...

Confusion between Torque and Torque Constants

You question is on how to find torque constant, but our discussion so far only talks about torque, not torque constant. For a particular DC motor product, its torque constant is a constant, a fixed value, a ratio based on the architecture of the particular motor.

On the other hand, torque (not torque constant) is a variable, depending on a couple of things. In other words it is a (dependent) variable depending on other (independent) variables. The following graph shows the independence.

Now, how to obtain torque constant from the datasheet.