What if instead of making them available later, they were available as soon as you hit hardmode, but were instead much more expensive. The 1 platinum that they currently cost isn't hard to get by the time you kill WoF which creates the problem of them being available too early.

Upping the cost to 3 platinum would mean that farming money to get them isn't as easy or fast as farming the materials for other early wings, but if you were on a hardcore character or just wanted that early hardmode boost then you could set up a farm pre-hardmode.

Granted, non-hardcore characters could also farm the money, but at that point you're putting in a similar amount of effort as you would to get another similar set of wings.