I’d move on. Likely management is on a level aware of the situation but is in a combination of denial/unwilling to change. If I had 5-6 designers leave my department in a short period of time, I’d be sure that something was up. Most of the time, awareness is not the issue, it is the will to do something about it, and your letter won’t change that.
In most cases like this, the messenger is shot. It will be easier to say “That Taftos guy was a jerk eh? First he leaves us in the lurch, then he sends us a letter saying how messes up we are!” That it will be to do something about it.
The silver lining is that you have moved on and hopefully you know better how to spot a place like that in future interviews. Hopefully your new position is a better environment.