Same as the model of all sources and CMZ as a ring in Fig. 4, but setting the CO-to-H₂ mass conversion factor to one order of magnitude lower than the Galactic value. We computed models considering the same previous dynamics and CR injection (brown squares), ten times more CR injection (orange circles), ten times less diffusion coefficient outside the CMZ (light blue x-marks), ten times less diffusion coefficient inside the CMZ (green triangles), and ten times less diffusion coefficient inside and outside the CMZ (purple diamonds).