Never spend coupons outside fuku event.
Spend in konoha tree, great plates or lucky board if you can because when you do you're getting passive power every time you spend, and then you will get whatever redeemable prize you wanna have.

What i would do since is the start of the server, get your carry already, with 20k you can get edo minato for example, but that depends on your likes man go check ninja list and read 'em up, then go for the one you like. If the server would be more advanced or you would already have 1-2 good ninjas then my advice is go full power spend.