If you think it will bring you peace, I would, but with the knowledge that she is likely to yet again disappoint you by either not responding or minimizing your trauma and her behaviour/blaming it all on you.
I have a similar mum and I recently found out she was pregnant again (she is in her 40's) so I did take the time to write to her about my experience as her daughter, not because I expect (or want) her to do anything about the baby, but because a small tiny little part of me hope that she will read and somehow self-reflect and do better, she can't fix me but she can avoid unecessary trauma to yet another child.
My mum is definitely a bit of a narcissist though and it's pretty much guaranteed she has read it, deleted it and will use it to further preach what a terrible daughter I am.
I didn't write it because I needed her validation or cared for her opinion, I wrote it because my mum is someone who will twist and change stories and conveniently forget about hurtful stuff that happened and if it was to be the last conversation we had, it was to be one where she had to face up things she so conveniently forget in her narrative. I was sexual assaulted as a child and teenagers, my mum didn't believe me, and then self-diagnosed me as schyzophreniac (despite me having absolutely zero symptom of schyzophrenia) amongst other things and completely brushed off anything that happened to me and her appalling behaviour (including my brother calling me names related to my assault and her just standing there). I don't know if she is expecting a boy or a girl but I truly shudder if it is a girl and it brings me a lot of anxiety to know that this innocent kid might go through the same, yet I also feel a tiny bit of jealousy that she might actually do better.
Anyhow, I get you OP, I totally understand never wanting to give up hope that our parents will turn into the loving parents we want and so desperately need. My mum came to therapy ONCE, she hated it and swore never to go again because the therapist told her she could probably do a few things differently but "how dare she tell me what to do and how to raise my kids" so yeah, therapy isn't for everyone, narcissists probably won't benefit from it at all, as they can't handle their behaviour being questionned at all.
Write the letter, send it or burn it but then forget about it and see what happens. If nothing at least you have said you piece if miraculously she decides to reach out and change, proceed with caution and see where it goes.
I feel relief in having been able to tell her how her behaviour regarding my trauma affected me, even though it has most likely fallen into deaf ears. I was only surprised that I also felt guilt for being honest, which in turn made me angry because only toxic parents will make you feel guilty for being honest and in touch with your feelings.
Do what's best and most comforting for you, regardless of your mother, OP and hope you get the results you are hoping for. x