SuzanneL60 I can't speak for your granddaughters new adoptive family but I can say we are drip feeding info about our adopted son's birth family in and he is only 4, adopted last year. So the info may well be used even if the letter is not seen until the little girl is older.
Our son was due to have letter box contact with extended family but it never got started and I am so sorry it did not.
The things I would love to hear if you were my son's paternal grandma are:
Things about your son when he was a baby, a boy growing up etc, just a few bits
Things about your contact with and experience of your granddaughter when she was a baby/younger
Physical things in your family like lots of people have red hair even if she does not, or freckles run in our family or we all have button noses etc
Hobbies or interests that run in your family like lots of people like swimming or do judo or whatever, or just all my kids or all cousins love the seaside.
These may seem like little things but they are a link to her wider birth family and to her past, which you are a part of.
I would also not aim to write the letter just once a year when it is due, if it is not too painful I would keep a word document or notepad where I jotted down any things that came to mind... eg. just remember my so wanted to be a train driver when he was a little boy.... etc. So when it comes to writing the letter it is not too hard to fill a page or whatever.
I am so sorry this is so hard for you and I hope you can feel proud that you are doing this for your granddaughter and also for your son, and, in time, may you feel some benefit to you.
Best of luck.