Sorry, expandable menu in the sticky menu is not practical since clicking on the mobile menu icon will only expand the menu in the header while you would have navigated far down below. Users will be confused if we jump suddenly to the header upon clicking the expand button.
Back to the top is a better solution from usability perspective. If you need sticky menu, my advice would be remove the expand button from the sticky menu for mobile, probably remove the logo also to address space constraints and just have a single line of navigation for mobile. Thanks