Almost a year ago now, we bought a set of tires and wheels to use on our trip through the Canadian northwest on our way to Victoria.
Probably wont get used until next year. Not sure how old they were when we got them from MiniSpares, but Yokohama seems to make the 008 in batches. I need to check the date code.
They have been stored in a shed in a black plastic bag.
I am guessing there is not much else i can do at this point for storage.
The main thing is to keep them out of the sun, covered and best if it's a cool dry place.
Storage of tires is as Dave says: out of the sun in a cool, dry place. Preferably, laying flat.
From your description, you have been doing it right, so should be okay.
Looks like August of 2019 is the date code. So they were eight months old about when I got them last year.