The letter won't get the attention you think it deserves. You will eventually get a phone call from a couple elders asking if you want to meet. They want to make sure you are sure and of the right mindset to make the decision you have made. .what you decide there is up to you.
Good luck.
I regret sending my letter. I feel like I could have done a lot more good fading and working under the radar. If you have any hopes of slowly working on friends or family to see the cult for what it is think hard about falling into the trap of disassociating and playing right into what the organization would want from someone who has found the real truth.