I have not used a Q2 monochrome, but own a Q3 28 camera, and use the monochrome jpeg option when it suits me, or convert the DNG file to monochrome in post-processing. Compared with the Q2 monochrome, the Q3 monochrome would have the higher resolution sensor at 60MP, and larger in-camera digitally zoomed files e.g., 35, 50, 75, 90mm, or whatever one chooses to crop in post-processing. That is anathema for many, but as a confessed zooming sinner who will end up in Dante's 6th circle of hell for heretics, I find the zoom files useful. Anyway, another useful feature of the Q3 cameras is the in-camera battery charging via USB-C port for connecting a power bank. Charging is fast, and can be done with inexpensive, small power banks in your camera bag; much cheaper than accumulating Leica batteries. It's faster than using the Leica plug-in external battery charger. And there is the flip-out LCD in the Q3 (and M11, and SL3) cameras, useful for low-to-the ground shooting.
Is all this worth a $7,300 upgrade from a Q2 monochrome? If I owned a Q2 monochrome, I'd likely not upgrade, even if I won the lottery.