Would we be wrong not to start letterbox, as so much time has passed and LO has never really had much direct link to birth family?
Our two have a similar start to yours. Like everyone else, I would strongly encourage you to write.
I'm adopted and would very much have liked more information about my birth family and perhaps the chance to get in touch with siblings. That does not in any way detract from my relationship with my real parents, it is more about curiosity and identity than anything else.
If you agreed to do letterbox contact as part of your adoption process then I feel morally you should follow through now you have the opportunity.
I think the reasons given for writing by chocolate and others are spot on, but I slightly disagree with this bit. Ultimately the choice is yours about what is best for your family.
As Sanders said I would do one letter a year, without photos or pictures from you. Assume anything shared with FC siblings will get to BPs. I would not share with your children that contact occurs until they are much older and able to understand and process the issues. Be prepared that BPs often don't reply. There is a really difficulty balance about when and how much control they get over the process without it upsetting them- no one can tell you how to do that, it's about your family.
I would keep throwing in little drips about adoption, so you can be open if they have questions and they have a general sense of it, but try to keep life story discussion low stakes and normalised, even if it doesn't feel like that to you.