Just my observation - if your letter shows too much anger, irritation, annoyance, whatever, it still looks to your H like you are very "involved", while he seems content to deny you his involvement.
I think, perhaps, if your letter comes across as cooler and less urgent (and therefore needy), H may come to the conclusion that you are happy and ready to move on, and this may jog him into action a little sooner.
While you say you would accept either answer from your H, it is obvious that you would prefer him to work on the marriage than not, or you wouldn't be on this BB, right?
Livnlearn
"The unexamined life is not worth living" - Socrates