I considered a “tool bottle” type of storage for when i dont use backpack, but i choose to use a Lezyne Road Caddy saddle bag, but instead of it on my seat, i mount it under the top tube on frame, it mounts easily with its velcro strap, is quick to remove if necassary and slim enough i dont hit my knees on it when pedaling. I also use a bottle mounted frame pump in addition to the bottle cage and then strap a tube to frame for longer rides if needed.
Another option i considered was the SKS Spacecage, which is a water bottle holder with small storage canister on back side but storage was too little for me to make use of it.