I managed to get Onboard working on wayland. Kubuntu 25.04
#1 Edit the shortcut in the menu. Within the KDE Menu Editor look for the Environment variables field and add “GDK_BACKEND=x11”.
#2 Go to Onboard preferences page. Under Keyboard – >Advanced set:
Input Options → Input event source: GTK
Key-stroke Generation → Key-stroke generator: uinput
If you have time please try it. I would like to know if it works for others too.