If you want something strapped on the back of a rack that is a backpack, I would be more inclined to use one of the drybag type backpacks. There are several of different sizes and brands. I have had a Seattle Sports one for over a decade, very simple, roll closure at top with two shoulder straps. It was more simple than I wanted, the two top buckles were intended to buckle together for closure, but I sewed on side straps with some more buckle parts for that which gives it a bit more volume. It is the blue pack in my photo, I estimate at about 27 liters, tips the scale at 670 grams.
Disregard the yellow pack in the photo, that is too big to strap on a bike with a rating of 70 liters. The photo of these two packs is from one of my canoe trips, the yellow one goes behind my seat in my solo canoe, the blue one up in the bow, I can shift the blue one fore or aft to balance the boat. I had nine days of food and much of my camp kitchen in the blue pack at the time of the photo.
That said, I would use small panniers, but I will not try to convince you to change your thinking. It sounds like you really want a back pack on the rack.
Not much of an opinion on the front packs. They look interesting but you are locked into a system where if you had one of the various Anything cages, you have more flexibility on what is strapped to it.