StereoRectifyUncalibrated calculates simply planar perspective transformation not rectification transformation in object space. It is necessary to convert this planar transformation to object space transformation to extract Q matrice, and i think some of the camera calibration parameters are required for it( like camera intrinsics ). There may have some research topics ongoing with this subject.
You may have add some steps for estimating camera intrinsics, and extracting relative orientation of cameras to make your flow work right. I think camera calibration parameters are vital for extracting proper 3d structure of the scene, if there is no active lighting method is used.
Also bundle block adjustment based solutions are required for refining all estimated values to more accurate values.