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 Ossa 48L is a framed pack can be stripped of its frame and used frameless also.
The Sonder 36L is a frameless pack, it does require a little back panel structure to keep the pack collapsing on itself and to help protect your back from the pack contents.
If you use an inflatable mat you can fold it to the dimensions of the back panel or you can strap a sit pad on the outside of the back panel. Sewn in webbing loops and bungee cord are included to set up your sit pad and are standard on all packs.