Hi Martine-
Meeting with selected members of the senior management team beforehand and delivering a coming out letter A) controls the spread of information and mis-information, and B) controls the number of people that know, and C) allows time for your message to be spread in an appropriate manner, and D) may provide certain legal protections to you.
Here's how I did my coming out at work.
I met with 3 key people (the CEO, the HR lady, and my boss) individually one day in guy mode and gave each of them a coming out letter and talked to them, describing how I had gotten to this point in my life. The CEO later spread the word to other key management folks. The CEO asked how the company could help and I suggested that they get a gender therapist to come in and give a presentation to everyone on one of the days I was taking off prior to the change - that way, people would have the information that they needed and inappropriate water cooler talk, outing me, or misgendering would be reduced. People could get their questions answered, and I hoped it would help with concerns about the restroom issue.
My boss was the only one uncomfortable with the idea of my transition so I came up with an idea to "work from home" and meet him and the senior management team as myself at a local restaurant one day to help ease concerns. He was ok with that idea and I took it back to the CEO and she thought it was an excellent idea. I think he feared that I would look like a garish drag queen or something

On the day when I was to meet the senior management team at the restaurant I dressed as nice my wardrobe would allow. 7 members of the senior management team (3 of them owners of the business) showed up, and after some initial awkwardness they warmed up and we had a very nice lunch. I was able to share my story with them and I got their support.
I took a few days off and during that time the company wide meeting happened with the gender therapist. I would have loved to have been a fly on the wall at that meeting LOL.... but I did get feedback that it was a great, informative meeting and people were able to met with the therapist one-on-one afterward to get their questions and concerns addressed.
On first day back at work - I got up early, got myself ready, drove to work, and confidently strolled into the building. I had beat everyone into the office so I just left my office door open and waited. As people arrived they would visit me and would offer words of encouragement, and of course there was the peanut gallery that strolled by gawking at me

In the year since I came out at work there has been no inappropriate water cooler talk that I am aware of, very few misgenderings, no inappropriate questions, and no issues with using the ladies room. I am stale news around the water cooler at this point and life goes on. I am accepted as a female and I have even experienced some misogony at work which tells me i've arrived at my destination.
My coming out experience at work could not have gone any better.
Good luck on your own coming out at work