Keep the flash sales.
Add every accessory and costume set to it - this includes all accessory color themes. Continue to have sub-750e accessories. [So there is something for everyone]
Add every accessory and costume item to the iron lockbox. Buff the unboxing from weighted towards sub-750e to truly equal chance. [So that Unbox is competitive with Supply depot]
Unless you were planning to do something like this already, It's clear that you have been tinkering with 'cosmetic drop system 2.0' for a while now.
While the flash sales are nice, I feel like taking a glance at these videos for some rather interesting points on why some things should be scarcer than other in a Free-To-Play game, and why everything shouldn't be completely equal chances.