Hello Felix,
you will also need to tune the controller. You should tune the controller first in a simpler system with a constant torque load until you can get the motor to spin at your desired speed there. Then you can add the rest of the model. In your model, there reaches a point where the motor can't track the torque request generated by the speed controller and that is where your speed flatlines. You also may need a higher dc voltage to drive the motor to the speeds you want. I couldn't tell from the data sheet what your motor was designed for.
Regards,
Joel