Game Mode

Option 1

unset LD_PRELOAD exec startplasma-wayland --xwayland --x11-display $DISPLAY --no-lockscreen --width 1280 --height 800 -- plasma_session

Option 2

I don't know if you still need help with this but maybe someone else does. Konsole is a Qt application so it uses the standard Qt command line options to set window geometry: LD_PRELOAD=${LD_PRELOAD/_32/_64} QT_SCALE_FACTOR=1.25 %command% --fullscreen --notransparency --new-tab --hide-menubar --qwindowgeometry 1024x640

I scale it from a lower resolution because that's what Valve does with the default Google Chrome installation. I disable the menubar because navigating menus is too hard anyway. The string substitution on LD_PRELOAD is to prevent the linker from loading the 32 bit version of the steamoverlay library, which would cause a wrong ELF class error since Konsole is 64 bit.

I did the same with Elisa, a media player, which is also a Qt application. However, Elisa is a Flatpak, so you have to set environment variables with an option: run --branch=stable --arch=x86_64 --command=elisa --env=QT_SCALE_FACTOR=1.25 --file-forwarding org.kde.elisa @@u @@ --qwindowgeometry 1024x640

I don't do this anymore because I ended up pairing my phone via Bluetooth to play music on the Deck. It's still useful to know because there are other useful Qt applications that you might want to apply this to, like KeePassXC or Kate.