A menu is something that appears on the menubar at the top of the screen - so what you have there is not a menu and so KM can't use it with it's in-built action.
It would be cool if this strikethrough macro could work as a toggle, i.e. switching strikethrough style on and off.
This would require a If-Else-Conditional and the ability to detect the styling properties of the text contained in the system clipboard. The latter isn't yet possible, or is it?
Yes, I toggle, but with another app. But KM can certainly do that too.
I don't think so, it's just a matter of alternating between two actions.
If the "cycle" starts with strikethrough, but the text is already strikethrough, then I trigger the shortcut again. It's not perfect, but it's enough for me.