Inferring back emf from oscilloscope
This is a big stepper motor. The motor inductance of 8mH per phase is an indicator that is made to be used with high voltage stepper driver, like 325 VDC or 230VAC rectified, with a chopper driver, that has a current setpoint.

Have a look for similar model: Sanyo Denki



It has near 4mH per phase, similar to yours if you connect 8mH phases in parallel and it has 0.46 ohm per phase, similarly yours have 0.9 per phase, but connected in parallel gives 0.45 ohm.  With a BEMF constant of 161V/kRPM you can expect similar torque characteristics also from yours motor.

At 140 VDC supply it has a knee point at 1200 rpm

At 48 VDC this drops to a 300 rpm

EDIT:

143mv/RPM is :

$$\require{cancel}\dfrac{143mV\cdot min}{R} =0,143V \dfrac{\bcancel{2\pi\ rad}\cdot 60s\cdot\bcancel{ min}}{2\pi\ rad\cdot\bcancel{ 60s}\cdot\bcancel{R}} = 1.366 \dfrac{V\cdot s}{rad} $$

$$k_t \approx 1.366\ Nm/A$$

While this doesn't hold for your motor : 1.366*6= 8.1Nm, it holds for Sanyp Denki motor. 0.161V/RPM = 1.54 Vs/rad; 1.54 Nm/A * 6A = 9.2 Nm.