hey there NC, there is also a waterproof seatbag by arkel that looks like a carradice, look them up, might be a lighter, waterproof option.
for Ortliebs, the Plus models do weigh less than the classic material. Do a search for the ortlieb chart that shows the weights and voloumes of all of their products.
I still find it handy to have 4 panniers to have ample room for extra water and food.
Oh, and there are those more simple but light city panniers, no inner pouch and simpler but you cant attach a rackpack like I do.