I too wanted to send my xagf a letter. I think we all fantasize that somehow, while they read the letter, it will all sink in, they will repent and get better.
An alcoholic who hits rock bottom because of a letter is not an alcoholic.
In your letter a section of it was an ultimatum. If you are going to give him an ultimatum as a last resort I would suggest a clear one with measurable milestones, something realistic, and consequences with which you actually follow through if he fails to meet. Boundaries and actions are so important.
Sounds like you are doing what you can to keep your kids safe, good communication with them is key!