I'm a still a PIMO. Working on waking up my family - as they're dear to me.
Had I not have the family trapped in - I'd quit the same day I learned the 'Truth' is BS.
From my experience - it's super hard to change another person's point of view even on a minor issue. We, humans, tend to be stubborn, you know.
If you don't care much about getting your family out - go ahead with your plan.
I guess, social media could have even more impact than the paper letters. But in any case, the elders should be the last people on Earth to learn about your decision (they'll still learn it super quick, anyway).
Using a platform to voice your concerns and reasons to leave is also an option (but here, you'll have just few seconds before they turn off the mikes).
I'd vote for making some video with your explanations - and posting it on Youtube/Facebook, etc.. From my experience, even the hardcore Jdubs could watch it (they do it just to reinforce their belief they're better than you - but who knows...).