IMO that really depends how much you need to have a conclusion to this - if you can live your life happy but with the door open a teeny crack then I would say no cos the goodbye letter might seal it for your W, although I understand you must feel that it is already sealed from her pov. If on the other hand, you need to bring things to a conclusion I think a letter would likely achieve that - but there may be no going back from that position.
Perhaps you are thinking it could have a positive effect - of getting her to take a look at the reality of the situation? I'm not sure it would do that.
Best - GFI
Me: 40ish W: 40ish Together: 20 ish years Married: 10ish Years