Not a particularly good point. Just because gender and race are both social constructs doesn't mean that both run on the same ruleset. Women and men exist as social categories in all cultures and trans people are seen across cultures existing even when the idea of trans people is buried and obfuscated. In reality there is no %100 male or female body. Our bodies are all mutable holding different measures of the same horomones, the organs are basically just inversions of each other and male and female are just sliding scales of intersexual potentialities. Trans people in many instances aren't just looking for just a skin deep social category change. If it were a possibility a lot of trans people want to be able to have the full biological function of the category including the ability to birth children. While gender is performative transness is not strictly all about affect. Gender performativity as described by a few genderpunk philosophers does not well explain the full phenomenon of transness.

"Cross racial transness" if it exists in good faith at all, is pretty bloody rare. The likes of Oli London have found their way into the feeds of the anti-trans pipeline where they have used their experience to try and make gender performativity and trans people look like made up bullshit. As a community this has been used as a cudgel against both trans people and POC. Catch all rules for every instance of social construct does not exist - each one is unique and the negotiations are complex.