Pulling the plug on toxic relationships is a real thing. Easy for some; hard for others. And there is no guarantee that your path in life improves, but at least now it's just you and people you WANT to be with, if you do find such.
C-D, you have to do what you have to do, but hear me out. Your parents don't want to hear the details of your change of belief because it would threaten their own belief. My advice would be to keep it super simple with them -- "I changed my mind." Don't let Mormonism rent space in your brain.
Granted, they will likely never be able to know that part of you that changed your mind. But there are lots of other parts of you. I can tell you as a nevermo that even non-Mormon parents will always have some blind spots. While maddening, you can find ways to work around those blind spots.
My personal opinion is that blowing up a family relationship should be done only as a last resort. Create distance if you need to, sure. But life is long and people can and do change. I strongly urge you to reconsider sending this message.
Your relatives and mine sound similar, Cold-Dodger. The only way to deal with toxic people is to cut them out of your life, even if they're biologically related.
And, if you haven't learned this already, it takes more than blood to make someone family. I've been treated better by some of the shipmates I had in the Navy than by my own relatives; I've even thought of changing my last name to the surname of my unofficial older sister. Honor the people who treat you like family.
Remove the phrase about almost losing your will to live.
Such an admission in writting may come back to haunt you if your family decide at some point to challenge your competency or to side against you in a custody case.
The old "anything you say can and will be used against you" thing is always relevant.
Those are for sure the last sentiments you want to come from you, but, the words are the wrong vehicles to convey something this important.
Rewrite and rewrite until the message wreaks of the class and elegance deserved. See if you can resurrect Mark Twain to be a ghost writer. Look up some bon mots and quotes and craft some phrases that will make them stop and think something other than, "Wow. Satan really *has* got a hold on CD."
Out-class them. Profanity is not your friend in this instance.
I found it necessary to confront a BIL once about some bad behavior toward my mother and was ripping him a new one. Went well until I used the eff word and then that was all he focused on and used it as a way to discredit everything I had said previously. Nothing counted anymore. Well it did, but he used my use of the eff word as a tool to turn the conversation in his favor in his mind.
I wrote a letter like you are about to write to my brother the most arrogant MP/Area Authority/Temple President/Apostle wannabe in history. I spent weeks on it. I sat on it til I was sure it was exquisite. I sent it. Never heard a word. Which I expected. I wrote it for me.
A couple of my favorites whose essence can spawn words to sting--in a good way:
"Insanity in individuals is rare--but in groups, parties, nations, and epochs it is the rule." Neitzsche
"You have enemies? Good. That means you have stood up for something in your life." Winston Churchill.
"A nation of sheep must in time beget a government of wolves." Edward R, Murrow.
And my very favorite . . .
"To a man with a hammer, everything looks like a nail." Mark Twain.
Don't know if it would be your style, but just like when I used to do the two-and-half minute talks in SS, I would take the meaning of a quote and rethink it to make a statement about something else. Substitution. Building on a thought.
"La gatta frettolosa ha fatto i gattini ciechi." The hasty cat gave birth to blind kittens. Or, doing something in hast can result in less-than-perfect results.
Have more than you know. Speak less than you show. Anon.
Narcissistic Resistance https://www.youtube.com/user/mattshsq and Narcissistic Survivor are good places for some empathy commiseration and wisdom on dealing with toxic family. The only way to win is to not play the game (i.e. no contact). You're doing the right thing.