It's image scale that matters when imaging small objects, not just the size or focal length of the telescope. In fact, given the seeing conditions that most have to deal with it probably doesn't do much good to go beyond eight inches in aperture in terms of the detail that you can record with a camera (you may find that a bigger but blurrier image isn't much satisfaction). That said, for any given image scale a larger aperture telescope will generally produce a "faster" system in terms of exposure. But, that also depends upon your camera and its read noise and quantum efficiency.
Thus, you should be thinking about BOTH the scope and the camera and for the "best" results you need to match one to the other for any particular image scale and type of DSO. And, of course, once you change image scale you need to consider how well your mount will perform, since poor guiding could negate any change in image scale or aperture.
Your signature says that you have access to a 14" Newtonian and a 9.25" SCT and both of those should make a completely satisfactory scope for imaging galaxies.
If you just want more image scale then one inexpensive option would be to change your camera to something like Sony's IMX178 or IMX183 (both with 2.4 micron pixels). A change to that camera from your current ASI1600 would give you an image scale boost of 3.8um/2.4um ≈ 1.6X which would be similar to taking your MN190 to a focal length of 1600mm (with an image scale of approximately 0.5 arc seconds per pixel).
However, if you are looking for the ultimate galaxy imaging system then you probably want the largest scope that your mount can honestly handle with the largest pixel size camera you can find in a combination that will give you something near to the same 0.5 arc second per pixel scale that you could get by just changing cameras on your current MN190.