You in an incredibly difficult position. But I really am happy that from this Friday, you'll be on the path to reclaiming your identity.
I see you're writing a letter to your dad. I wonder if there is a reason for that, as opposed to a face-to-face meeting? Sometimes, having these difficult conversations in person is better than a letter.
About the conversation or letter (whichever you choose to proceed with), while the main aim is to tell him you are trans, how about also telling him how much you trust and love him as your dad, and that you hope he will be able to accept your decision once he has time to process it? There is the chance he may take it badly and that's something you'll have to mentally prepare yourself for. But one bad reaction here doesn't mean that all is lost. People have difficult relationship with their parents for a variety of reasons. Sometimes, over the years, they reconcile, and that's something you could work on as time goes by. He is transphobic now because of ignorance, but maybe that ignorance is because of his unfamiliarity with what being trans is about. And as he gets used to the idea that his child is a trans person, he may begin to realise you are and have been the same child he had always.
I hope you have a good support network around you, DireVi. If your dad does take it badly, please confide in these people.
Feel free to share how it goes in this forum as well, our members may be able to offer support too.