I don't know if it's possible to re-define the shortcut keys, but as a work-around:
I too find it very awkward to do Shift+Fn+N especially while lying in bed in the dark.
Instead, I do Fn+Bksp (to open Settings) and then Cosmo Settings and then use the slider at the top.
I'd rather have an easier keystroke also, but for me this feels the more convenient option until there's a better way.
Now I just wish that the Android settings menus would respect my Dark Mode config setting. Sometimes I use the "invert colors" option but it isn't awesome when there are graphics.