I can see good arguments for all of the milestones mentioned. And, I have stop counting birthdays of ANY sort, being of a certain age, as they say.
But if I had to give ONE date, it would be the date of the letter from the Doctor saying, "Done." (Although she took way more words than one to say it.)
When I changed the gender marker at work, HR asked me for the "As-Of" date, I told them it was the day I was born 50 some years ago. They said, No, the official date when it changed. The date on the letter seemed highly appropriate...

Kate