The letter is a good idea unless she uses it as blackmail to divorce you but you should know your wife better than I. Would she do that? Be sure to address the most common question SO’s have which is, “Are you gay?” followed by, “Do you want to become a woman?”.
I was deep in the closet for decades until I came out to my wife about six years ago. One of the first things she said after “Are you gay?” was, “This explains a lot.” After a lot of questions and long talks she has come to accept more and more of Michelle.
You talk like you think Darcy can just go away if the wife doesn’t approve but I don’t think its that easy, at least not for most of us.
I suggest you ease into a conversation with her about crossdressers and see how she feels or a more dramatic approach would be to leave some feminine article out for her to find then have the conversation. I think either one of these methods would be better than having her walk in on you dressed or to find your whole stash without any prior clues. She is going to be pissed no matter how she finds out by the fact that you kept this hidden from her.