In none of the screenshot can we see you adjust the scalar value of the material instance to be honest - it’s a very strange method and the Set Brush Resource to Material diamond by-reference pin is not connected. Also, consider looking into Set Members In Struct node - there’s generally less wires and less room for error:
But to the point, it’s all much simpler than that:
Create it inside the widget and assign to the element that needs it. Make sure the material operates in the UI domain, of course. There’s no need to fiddle with the brushes.
edit: Here’s a spot on example: