Hi Ben.
Nice letter. As my own mother lived a couple of hours from me at the time, I wrote too. It gives you time to think about what you are going to say and gives her time to organise her reaction.
Perhaps you could include something about Gender Dysphoria being a condition caused by early brain development , cause unknown. That way you can absolve her of any blame and stress it is a medical condition, not psychiatric.
If she does want to contact me I would be happy to chat, having a son who transitioned several years before me. His actions showed me the way, so I have seen Transition from both sides.
Hope it all goes well.
Karen.