@op you are basically asking for something that already exist ingame. A gear vendor that changes male gear into female and vice versa. But you had to write an essay about that suggestion :P

While it's true that you don't get the achievement if you aren't lvl 60 yet, doesn't really mean you run into storage problems, cause like already pointed out, you only need to keep the sets for the classes that are not 60 yet. The rest you can dumb and repurchase at the salvager npc for BOTH genders, no matter what your current gender is.

The event is live since years... If you really don't find the time to level, there are always jump potions if you care that much about the achievements.

You have 400 slots in the glamour dresser, 2 free retainer. What are you hoarding?