Hi OP, not sure exactly what you've been through, but you have my sympathies.
I am now NC with my dad after many decades of angst and (IMO) his poor behaviour.
When I attempted air-clearing/closure with him, I bought him a small gift (£10 bottle of wine he likes) and took it over to his place at a time we'd agreed to have a cuppa.
I was super worried and anxious but I knew I needed to get a lot off my chest finally, so I went though with it.
He did not love the chat, told me I was 'playing the victim card as usual', did some of his typical DARVO... He tried to twist a lot of what I said and because I was there in the room, I could just hear him bluster and then say: "I didn't actually say that, dad, what I said was..." And stand up for myself, which I couldn't do as a child.
It was draining and not fun, but I was able to persist and we did sort of get somewhere. I remember leaving and taking this big deep breath in the cool evening and I felt physically lighter.
Obviously the situation then deteriorated again and I ultimately chose to go NC.
If you can create a situation where you'd feel safe to do so, then I would recommend trying to say your piece in person vs a letter. You can make sure that they really hear you - that they're not twisting it or deliberately seizing on an irrelevant bit to deny what you're saying...
Giving them a letter would feel to me like giving them control.