you gotta say what works for you and are to be congratulated on the your decision to tell them to f*#k off 
of course it will make it easier for them to thought stop the actual reasons for you leaving.
One thing I would change is:
I agree that people should not be “luke warm”. You either believe everything 100% percent or you don’t. You can not sit on the fence, and I have decided not to anymore; therefore I am submitting this letter of resignation so to speak.
Why can't people be allowed to be lukewarm? Cults deal in black and white thinking. The real world deals in shades of grey. It's called using your own conscience - not an implanted one dreamed up by a pulblising and property company from New York.
Maybe something like:
"After much prayerful/thoughtful* consideration I have come to realize my God-given conscience had been slowly eroded and replaced with a organizational counterfeit. My original conscience has no problem enjoying normal loving family relationships with those you have decided are worthy of extreme shunning. My original conscience has no problem enjoying my birthday - Job 1:4. My original conscience does not find the need to count and report the time spent on distributing Watchtower literature. In fact I have searched the Scriptures and cannot find the requirement for a publishing and property company based in Brooklyn to be an intermediary in my relationship with God."
*delete as appropriate
Anyways, welcome to JWN, welcome to the rest of your life and enjoy your new freedom.