[[ https://lxr.kde.org/search?_filestring=&_string=CTRL.*ALT.*Comma | Used https://lxr.kde.org ]] to make sure no existing software uses this shortcut for anything.
Tested in KDE Neon. Tried out Plasma, KWin, Dolphin, Kate, Konsole, Gwenview, Okular, Konversation, KTorrent, and Skanlite; all now have a consistent keyboard shortcut for their "Configure <Program>" menu items.