Humidity and oxygen are the big flavor killers for pre-ground coffee. CO2 valves are more helpful for freshly roasted whole beans than for pre-ground, which has mostly finished degassing by the time you open it. What matters most for you is a truly airtight container, minimal headspace, and keeping it cool and dark. I would go with
21oz Stainless Steel Coffee Canister. Stainless steel helps block light and slows temperature swings, and the 21 oz size is big enough to hold about a week or two of grounds without leaving a lot of air inside. Best routine that works in humid climates is simple. When you open a big tub, split it into several small freezer bags or jars, squeeze out as much air as you can, and freeze all but one. Fill the canister with only what you will use in a week, then store the canister in a cupboard away from heat and sunlight. Pull a new frozen portion when you run low and let it come to room temp sealed before opening. Avoid the fridge, use a dry scoop, and close the lid quickly. This keeps flavor lively far beyond a week.
