staehpj1

First, I think all thermarests can be folded in half before rolling up, even the very early ones. There may or may not be any advantage to it in your case, but you can fold them.

When using panniers the only thing that I pack on top of the rear rack by choice is the tent. Other stuff goes there only if it doesn't fit in the panniers. I don't use a ground sheet these days but when I did I rolled it up with the tent. I roll the tent with the waterproof bottom on the outside so no need to try to make it more waterproof. It is fine even in a hard all day rain.

I have always packed my sleeping bag inside.