This is a useful technique in therapy: write your letters, but don't send them.
I do that a lot on this board...
Anyway, have thought a lot about gb letters over the years. Sometimes I've wanted to write everything down, explain myself in large detail, and other times I felt like writing nothing at all. There's really just one person I'd address it to anyway, and that's my mum. The thing with people is that if they understood you in life they'll probably do so in death, too, especially if you've been able to talk to them about this issue before. Otoh if they didn't while you were alive I'm not sure this'll change once one is dead. Having the last word on it, literally, probably won't change much. It might lead to new questions/arguments by the recipient if they just don't get it, or they might feel guilty because they didn't listen to you while you were still here. So, I'm leaning more towards a short note in the classic style. Ofc, some may interpret that as cruel and unaffectionate. It's just crazy and wrong whatever we do.