I am lucky. I live far away from USA where everything is cheap, but I have one of the best car builder living in my town.
He has been working over 20 years with American cars, fixing them for living. Before that he did dragracing.
He told me that he wont do transmissions anymore. He can, but he wont. There is so much to know and so many things that go wrong even that you are very careful.
Other guy here does those, he has been doing it for atleast 30 years, maybe closer to 40 years. He is good and thats why he isnt cheap. The reason tranny fixing costs lot of $$$ can be easily understood.
I am pretty sure that there is smart/good vanners here who can fix their own transmissions.
I wouldnt do it...