Frameless packs aren't designed to carry heavy loads. Figuring out your base weight and your average weight of consumables will get you started.
The Sonder 36L and Luxmore 48L packs are frameless packs, they require some type of back panel structure to keep the pack collapsing on itself and help to protect your back from the contents.
If you use an inflatable mat you can fold it to the dimensions of the back panel or you can use the back panel foam option. Using either of these or both is made easier by adding an internal pad sleeve to hold things in place while loading your pack.