It goes way beyond the time off required. Oh, I agree the impact is less the further you are in your career. My point is that it's going to have a strong impact regardless of what age you do it, unless you're already retired. People often see their decades-long careers ended abruptly upon coming out. I've known people that's happened to, even within just the last 15 years. A few decades ago, a common piece of advice among some (particularly white middle class trans women) was to handle transition this way, even if you knew you were trans at 18:

1. Live as male.
2. Choose a career that pays as high as possible. Don't focus on what you want to do. Just focus on what pays the most.
3. Pursue a strategy of what would today be called FIRE. Think "work as a lawyer or doctor but live like a college student."
4. Make as much money as humanly possible while spending as little as possible. Reach early retirement money as soon as possible. Save up enough for transition as well.
5. Retire some time before 40 and only come out and start transitioning then.

In a time when job discrimination was so much worse than today, this was a strategy pursued by a lot of people. It was common advice to just assume coming out meant setting your career on fire. Of course there were people that came out even at 18, even 50 years ago, but most of them ended up in sex work.