Demonstration of how the recovered [Fe/H] gradient using our proposed correction works in g2.79e12 (left; strong bar) and g8.26e11 (right; no bar) from the NIHAO-UHD simulation suite, which are completely independent of the TNG50 galaxies that are used to determine the correction. As discussed in Section 4.1.2, we added 0.05 dex and a 15% error to [Fe/H] and age and then denoised to make the best extensions to the MW. The black line in each panel represents the recovered metallicity gradient with the correction applied, and it matches the true [Fe/H] gradient evolution of disk stars (red line) better than recovering the metallicity gradient without the correction (gray line).