You can't control what JW's do or what they call things. They say anyone that leaves "disassociates" themself. I call that absolute bullshit. I resigned. They are certainly allowed to call it whatever they want. Of course, as we all know, they lie quite a bit, so it doesn't matter what they call anything.
In your case, my thoughts for someone in your situation is pretty simple: Do what you want. If you told the elders all that you wanted, and THEY want a letter, THEY can go pound sand.
Write a letter only if you want to and it serves some kind of purpose for you. In my case, it served a lot of purposes for me. I knew who would read it at Bethel.
In your case, tell the elders very simply that you have done and said all you care to. If they insist in trying to talk you into it, repeat it ironically until they get the message.
Do things your way, for your reasons, and don't worry about things, people, or cults that you have no control over.
Btw, congrats on your freedom.