Consider how the militaries store minutions: In sealed metal tins, like sardines. Keep your flares in a DRY airtight container, and they typically will last for decades. Plastic is not airtight, it usually allows some oxygen through. Get a metal surplus ammunition can, which as a rubber gasket
and good tight seal. Clean it up, stow the pyros in there. That way if they catch fire by themselves--they are contained. No air, no fire.
No air, no moisture, no degradation. Stick an inventory on the outside of the can, so there is no need to open it (and add new air and moisture) for inspection
How long can munitions last? Well, a 1770's cannonball filled with DuPont black powder was recovered in the 1990's (200 years later) after being packed in dense mud. It detonated just fine, with the fuse replaced. And WW1-era ammunition buried in the deserts of the mideast is still being found--and used. No special storage
, just hot dry desert sand in former trenches.
Work every time? No, but if 80% of your old flares still work--that's a pretty good return on investment. With just 3 or 6 flares on board, the world only has what? Two minutes to see you?