I put my ammo in boxes made from card stock. I purchased them from Repackbox.com. They were around .23 cents per box including shipping. You get a pair of cotton gloves and 1000 labels with every 1000 boxes ordered. These are really nice boxes. I think that I got 35 boxes of 40 S&W pistol ammo in a 50 Cal ammo can. I printed the load data on the label and affixed it to each box. Repackbox.com also sells desiccants and mag bags as well. I ordered 4000 5.56 and 2000 each of 9mm and .40 boxes.
With the safe in the garage, it will take lots of effort to keep out the humidity.
Maybe insulate the garage well to keep the wild temp swings in check.
Put in a heated rod that is made for helping with moisture in safes. This heats up the inside of the safe, and makes the relative humidity lower.
Buy a Hygrometer (humidity meter). One with a High and Low memory so you can keep track of what the inside of the safe is doing.
You want to try to keep the humidity below 50% the majority of the time.
Don't leave the safe door open for long periods, this will allow the inside lining of the safe absorb moisture during periods of high outdoor humidity.
Mags loaded, extra in boxes in gi ammo cans with dessicant packs and loose in gi ammo cans with dessicant packets.
Springs wear out from exercise, meaning cycling, static state empty is same as static state loaded to a spring from what I understand but I am not an expert..