One who is transgender is one who wants to live physiologically as the opposite sex, literally an anatomically and chemically different version of themselves.
This isn't always true at all. There are many ways of being transgender. Many do not have surgery, many are fluid, etc.
Why do you think there are no physiological differences between races? Because there is a large body of research showing there are.
How does it follow that just because there might be differences in our brains, we aren't equal? Your brain and my brain are different, does that mean that we aren't equal?
It's been shown that male and female brains are different. Aren't we all equal regardless of sex? How does that square with transgender claims for 'brain sex'?
Supposing that some small difference between the neurology of a black and a white person, on average, was discovered. Would that justify transracialism? What makes you certain this is impossible? Would you bet $10,000 the difference between black and white brains is precisely zero?
According to your logic, a belief in 'brain sex' caused by sex difference must therefore make transgenders sexist.
Now consider that it is a fact that black people have larger brains, on the average.
Three time Nobel nominated anthropologist Philip Tobias (1970) compared 7 racial and national groups in a study on brain size/weight in which it was reported that the brain size of American blacks was larger than any white group (including American, English and French whites) except those from the Swedish sub sample who had the largest brains of any sample in the study. It was also estimated that American blacks had some 200 million more neurons than American whites, and brains that were reported to be 54g heavier (See Tobias 1970; Weizmann et al. 1990).
If transgender is justified by people having 'female like' brains, then transrace may be justified by whites having larger, more 'black' brains.