the most important factor in those cards is the elemental resistance, so you should level up those that grant you the elemental resistance you miss the most or that you want to strenghten the most. In my case is fire so i'm leveling up 3 fire sets of 4.

About the fact to not spend coupons now is not necessarily true. Yes, later we will have the scrolls in the sun and moon shop, but first we do not know when oasis will implement it and the fact germany has it right now is not an assurance we will get it within next 2 months, and second, is not that you have not to spend moon and sun coins to get them, so if you don't need to spend them there you can buy advanced threads and advanced refinements or other things you miss with what you saved up because you used the coupons now to buy them.