OP I think that the best advice for the benefit of your dc is to keep to your side of the agreement re letterbox and follow advice you got in training about what to include in letters, whether you get replies or not.
In terms of your dc's needs and wants, it will likely vary over the years, it is impossible to know at the moment. Adopted children have different needs at different ages in relation to understanding the situation with birth parents.
All children need help and guidance about human relationships, and it is not possible for adopted children take the lead in terms of contact with bio parents. I think you need to manage her expectations, and it sounds as though you need to know far more about the birth family than you do, to help you do that. Can you talk to the coordinator about getting more information?
If the coordinator has offered to help the birth parent write it sounds as though literacy may be an issue. There may be all sorts of other things which are causing the problem. It would be better that you had a clearer idea about this and you were able to explain to your dc - even if is to say her birth parent couldn't care for her and couldn't write letters but that you know the birth parent is reading the letters and so cares, for example.
Things may change for the birth parent over the years too.
I think that the very best thing for an adopted child is to have, by adulthood, a balanced view of their birth parents - and for most birth parents this will include good and bad. Not all, obviously, but most.
Ultimately it is down to you to find out what the situation is here.
Developmental trauma can also be lifelong and enduring it isn't if you seek the right help.
@jizelle I think it helps everyone to hear other points of view so keep posting if you want to.