Hi Simon,
Such kind of customization officially is not available in AutoCAD for Mac.
But if you are familiar with manual CUI file editing, you can easily do this.
Context menus information is in PopMenuRoot.cui file which is inside
"/Applications/Autodesk/AutoCAD 2019/AutoCAD 2019.app/Contents/Resources" folder (if you have 2019 version)
You can use plain text editor to remove / add menu items in context menus.