From the numbers you've quoted, you've tried at a very low acceleration and the motor is still stalling at a low speed. There's something seriously wrong if the motor can't exceed 150rpm - perhaps it's resonant around that speed? How does it when the motor is close to stalling? Have you checked what the corner speed for the 12Nm motor is on the drive voltage you're using?

As irving said, you can easily get a good enough approximation for the inertia of the parts from assuming they're cylinders and using the following formula:
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Where m is the mass of the cylinder and r is its radius. That will get the inertia of the pulley connected to the rotor of the motor, but you need to scale the inertia of the parts on the driven shaft by the square of the drive ratio, so in your case they will be 2.67^2=7.11 times lower than calculated, as I think you have the smaller pulley on the motor. The total inertia of the system is therefore motor rotor ineria+motor pulley inertia+1/7.11*(the rest). By all means work it out, but currently I think you're problem lies elsewhere, as with a very low acceleration the inertia will make little difference.