I need both "Apply scaling themselves" and "Scaled by the system", depending on the X11 app I am using. Some more recent X11 apps, such as Android Studio or VS Code, support scaling themselves, but a lot of older X11 apps, such as Qt5-based ones, do not support self-scaling. I have tried `export QT_SCALE_FACTOR`, but the mouse pointer pointed wrong items when trying to click a submenu of their main menu, and the window content was not painted correctly (they were blank until I resized or moved mouse over them), so QT scaling was not very useable. It's inconvenient and time-consuming to repeatedly open the Display Configuration and change it every time. I wish that toggling this setting was possible with a system-wide shortcut (Input & Output -> Keyboard -> Shortcuts -> System Settings).
This seems like a really niche case, also in order to propagate the new settings properly, you'll need to restart the computer. You could create a custom shortcut that runs a kwriteconfig command to update the config, e.g. `kwriteconfig6 --file kdeglobals --group KScreen --key XwaylandClientsScale true`