What would the average skin tone and facial features look like after 300 years if every partner relationship was interracial until there were no other ethnicities? Just a hodgepodge of DNA. What would the average human look like having a little bit of everything in them?
I just think the idea is neat is all.
You are just picking an arbitrary set of characteristics and calling it “race”. Nature has no concept of race because we are all genetically different and there is no line that one crosses from being one race to another. Race is purely a human construct, and a very unhelpful one, as it is just another way to divide us.