I cannot be sure - but I think the question has been up before - Search results for 'plasma on-screen keyboard' - Manjaro Linux Forum
I believe you may need to use some of the Plasma mobile packages - as Plasma Mobile can pop up the keyboard when it is needed.
If I recall this - there may versions with work with X11 but not with Wayland.
I remember years back I used an IdeaPad Miix 700 a tablet with detachable keyboard - it worked best with Gnome