My CRT is triple sync so, on windows at least, I ran GameEx at 640x480p and when launching a game groovymame would switch the resolution to the original game resolution. GameEx is looking good at 640x480 at least on the version I have. I haven't updated in many many years.
Not sure about running 15khz resolutions on Windows 11. I would have to make some research but I really want to ditch windows. With XP we could hide it quite well but I'm not sure we can do the same with recent versions.
I might as well wait until the multiplatform version of GameEx is ready.