You're projecting about a future that you don't know about and then thinking about what action you should take to keep the thread of hope alive. Your wife has filed. I don't see any mention of D being finalized. Maybe your W needs to feel like she has full control and full separation from you before she reinitiates any contact? Again, you don't know. You also don't know if you will never see her again or have contact with her again, etc. That's just projecting. She probably is still very angry (given the no contact & the moving ahead w/ the D it's a good bet). If that is the case, just letting the dust settle is best. You can always write that letter later after giving that some time. She's still not wanting pursuit, and your letter will probably feel like pursuit.
And, you are also assuming she has really moved on. If she really had, she wouldn't have given you the silent treatment. It wouldn't be that big a deal to her, and she'd likely want to check in. If she is still in pain, angry, etc., then she is still focused on you, just not in a way that allows for any connection. If she is pushing the D, she is probably hoping that severing that last thread will make her pain go away. It won't, but she has to learn that herself. Then she has to decide what she wants to do with that reality. So, is it over? Good chance. But you don't know.
Then the question becomes what to do about that? DBing isn't just about saving Ms, but about helping heal relationships. You clearly still care and still want hope, so keep DBing. She may never see this, but we only get to make decisions for ourselves.
If you really feel the need, write the letter, then stick it in a box that you will hold onto for a predetermined period of time. At that time, if you still want to send that letter, do it.
Just my 2 cents. In the mean time, I really am sorry for all the pain you must be going through and how hard things are. Not the way any of us want the story to go.
Hang in there.
Me: 50 W:43 S6, S3 M: 12 yrs. T: 17 M is bad & Not happy Bomb Mar '14 S 5 Feb '15 D Bomb 13 Apr '15 (but "no hurry") DB Coach May '15 Wants proceed on D Aug '15 Starting 1-on-1 negotiations Sept '15