You could just disassociate by life, rather than by letter.
Live your lives as you see fit, block every elder's phone number that tries to call you, tell them to get the hell off your property and shut the door on them when they try to come by your house, and refuse delivery of any certified letter they try to send you.
That's what we did.
We're still not officially disassociated or disfellowshipped (as far as we know). But word has gotten out we're apostate, so no JW talks to us anyways. The elders never got to the certified letter level, but we did block all their numbers and the one time some elders came by, they were elders that had never known us and I told them, "You must have the wrong house. No one lives here by that name." They knew I was bullsh!tting from the look on their faces, but they couldn't prove it.
If it comes down to it, they'll disassociate you or disfellowship you in absentia. But that doesn't even matter anymore, becuase it's just a bunch of Karens thinking they're doing something against you, when in reality it's nothing more consequential than a jerkhole former employer trying to fire you after you stopped showing up and moved on to another job and just stopped showing up.
Write the letter, if you feel it will be cathartic. But turning it in isn't really going to do anything for you other than acknowledge that you're still following their procedures.
It's been 5 years since we woke up, and it's all behind us. I consider myself an ex-exjw now...I like coming on here to give support to others, but that's the extent of my continued involvement with the cult. Now, it's just an interesting conversation starter at parties when religion comes up. Much less awkward to talk about your former cult with religious people than to talk about how you're an atheist. And usually, if your atheism comes up, they many times understand because of the cult background.