All the above completed and tested and works great with one slight issue.
It seems as though the flyout menu is opening behind the other menu icons I have on the opposite side of the page (Cart and Account). So the ‘X’ that closes the flyout menu appears directly behind them as they are visible.
Thanks very much for your comprehensive answer. This is quite a nifty little solution and gives me much more freedom within the header! Multiple trigger items is a great idea. I did see the flyout menu within the submenu tab but didn’t connect the dots.
This should definitely find a place within the knowledge base!