He said he needed to express his gratitude. That gratitude is important to express. I am sure they have issues and they find it very hard to write back. But I recommend writing the letter. If writing a letter to the recipients seems like a right thing for you, I would encourage you to write them.
Timing is very important.
When it feels right to you to write the letters, do it. I wrote as a gift to my mom. You are all giving me great incentive to express myself. Express yourself in ways that feel right for you. I think people mistake feeling healed with lack of emotional intensity.