I simply store mine within the retail containers (jars) it’s available in. At this time, I’ve 2 open jars around the kitchen shelf which i from time to time use for any single cup. Still good 5 years after purchase (and opening). Other medication is set aside unopened.
I have made coffee from instant packets (foil lined paper envelopes) sealed inside canned C-rations… contents over twenty five years old. Sampled all right. Also foil packets from MREs which were a long time (more than a decade) outdated.
That will lead me to believe (strongly) that bulk instant would easily store in buckets with sealed Mylar bags & O2 absorbers. As with every other lengthy term storage dry food.
Ideally, instant coffee would store nicely inside sealed metal cans (the type with metal covers rather of pull-tab foil), but the number of people get access to industrial canning equipment?
Plastic buckets may be simpler to move in a vehicle. They’d become more earthquake resistant than breakable glass jars. However nowadays, many of the instant stuff is available in plastic jars anyway. You can just stack plastic jars inside a card board box or plastic storage tub.
Unless of course you plan to purchase restaurant sized quantities, I’d just purchase the largest sized jars available and run with this. Around I love fresh beans, an eternity put in the Infantry trained me that instant is my preferred storage & carry solution. I did previously have a soft plastic 1 pint baby bottle of instant crystals within the field. The ultimate product just must be black, caffeinated, and hot. I ain’t that picky…