I'm planning on writing three notes to the same person and then one more less personal one to everyone.
A first one to say "I love you and it isn't your fault" to my sister but also "this isn't just selfishness; I've tried so hard for so long to keep going." I'll explain the details of why I chose to CBT in this one.
A second one to my sister to come out as transmasc, biromantic, and asexual and to try and open her mind up a little more about LGBTQ+ people because we've been raised religiously and I hope she'll take my words seriously because of how close we've been our whole life. I haven't been able to come out in life because I'm a coward and don't want to be seen differently, but I need her to understand for the sake of other gay/trans people and possibly her future kids if they turn out gay/trans because she's literally promised to homeschool her kids (to shelter them to keep them from hearing gay/trans people exist and "turning" gay/trans). I don't think she's bigoted because she's a bad person; I think she's bigoted because she's been indoctrinated religiously just like I used to be. This one is a little bit about me but mostly about other LGBTQ+ people because I want her to treat other gay/trans people better than she does currently and understand that we're human and deserve love and respect like everyone else.
A third one to my sister to explain my deconversion from Christianity. Our church is super evangelical and we've been indoctrinated our entire lives to believe everything in our lives should revolve around this hateful God and that we only suffer 1) as a test of faith, 2) because we're evil and deserve it, or 3) because we didn't have enough faith or trust God enough, which basically means we should be grateful for suffering and if we aren't, we deserve it. I want my sister to understand how this has contributed to my decision to CTB and how much I've tried and prayed for so long to just feel better. I don't want her to be angry or have unanswered questions but since I won't be around to answer the questions she has, I have to explain in detail if I want her to have a bit of closure and understanding of why I did what I did.
One last short letter to my entire family basically just to say "not your fault, love you, bye" because the rest of them don't deserve any more than that. It'll be no more than 10-15 sentences and most of them will say "tell X and Y I love them." The rest of my family will just see me as a victim of the "anti-God LGBTQ+ agenda" and not listen to a word I have to say, so what's the point of writing a detailed note to them?
All of the first three will be long. As a friend or family member left behind, I would much prefer a longer note than a shorter one. Not all people care that much because they're not that great of people, but hopefully my sister cares about me as much as I care about her and your letter recipients do too.
If they won't read your note just because it's long then they weren't the person you thought they were and didn't deserve the effort anyway. Long is never bad if you have a lot to say and need to write it down.