Hi, I can only say what we do and see if it applies to you but your instincts are right.
No block headings, that isn't a report, it's a letter. Be empathetic and compassionate but not 'emotional'; for example I would not go into how they might feel etc.
I'd write to child's birth parent by name. Either one letter to both names sent twice or one to each name. If the latter I'd give each parent same letter content. I'd do this because I wouldn't want one parent to have more info, and it's easier.
I date the letter and keep a copy.
If writing to one name, to be clear, I might into the phrase "just so you know we are also writing to (name) and you're both getting the same information....
We do write sort of chronologically but along the lines of themes but with no headings.
So we write about school, health, hobbies, holidays etc.
We might include a funny thing ds said or did or a clever thing.
I usually end with one or two questions. Our son was with birth family for a couple of years so I might say "Can you remember his first word, no worries if you can't."
Also I might include question replies if they had asked things, not personal stuff but general.
Also, I'd start by asking how they are and end by wishing them well.
Dh and I have bog standard names so I would end with our first names, best wishes from Italian and Mr Italian. This is because they were told our first names at the start, not even sure if we were consulted.
If they do not know your first names I'd be more cautious and might use a psydonyn or nickname (made up) especially if your names are unusual or you have high profile anywhere.
Names are more identifiable together, two common names less identifiable but one unusual and one common , more identifiable etc.
Some people might use initials. You could use your middle names, I usually like that as a sort of compromise, in all kinds of situations where I might be asked my name, not want to lie but not want to use my first name! Real but not regularly used!
I do not mention any other names of family or friends. I don't mention places we go to regularly.
I keep an electronic diary and this helps me to remember.
I also have an open letter on the pc to them. I start it after they send a reply or after I send a letter. I add bits to it as and when I think of it so that when the time comes it is almost ready. It makes it less stressful to me.
Hope that helps, feel free to ask me anything.