A comment on the bags that sit across the rear seat - for which I have to refer to a photo of my ST1300A6 almost fully loaded before setting off from a campsite.
We tour as rider and pillion. There is a stiff bag strapped on top of each pannier - using the grab bars to keep them secure. They also prevent most of the weight from being transferred to the pannier. They gently rest, rather than sit on top. There is a similar, but slightly smaller one on the right hand side. I once started to fashion a wider platform under the top box to support the stuff bags, but the experiments showed that they wouldn't work.
The problem is that even in the position shown in the photo - they restrict the rear view from the mirrors. I'm tall, so I get a slightly better angle. I can see what is coming behind from a long way back, and I can see if anything is close on my offside or near side. What I cannot see is anything immediately behind me. That is always a problem with (say) another motorcycle. But you have to make allowances for it. So I'd be using my mirrors more frequnetly for the distant view, and occasionaly wander in my lane so that I could check near rear left and near rear right. Anything much taller would or wider would give a serious blind spot.