Do you feel uncomfortable mentioning your reasons for estrangement?
We all are at different levels of estrangement or NC/VLC with our family of origin (it rarely sticks to one or both parents, the dysfunction is systemic and what role you play in family).
My question is do you fear that sharing your 15 year history and multiple step by step reasons for estrangement can be used to manipulate you.
For each failure they can easily say "we shouldn't have done that, you deserve better, we are sorry", or gaslight you by saying something like "this is not what happened, this happened".
I think estrangement letter is like you giving them lifetime ammunition to manipulate you so that they can restore access, authority and obligations.
If they respect boundaries and respect distance for several years then you can start accepting that they have changed or at least accepted a changed relationship status.