I believe you need to set the option for a virtual keyboard in your config, appended to the line in the [Wayland] section of your file. You do need to have the keyboard installed, it might not be on your distro automatically.
[Wayland]
CompositorCommand=kwin_wayland --drm --no-lockscreen --no-global-shortcuts --locale1 --inputmethod maliit-keyboard
I used this on my previous touchscreen laptop on the current Plasma 5 and SDDM, and in brief testing it did work. Not sure if there is a different name for qtvirtualkeyboard in Qt6/Plasma 6 compared or anything, if you wanted to try that instead of Maliit.
May not be valid for Plasma 6