This. The comics make it clear that a HUGE amount of intermarrying and culture mixing went on to the point that it was common for the same family to have both earth bender and fire bender kids.

Hell there is an assignation attempt on Zuko by an earth bender for his support of returning the colonies to the earth kingdom. They go out of their way to make it clear this isn’t an Israel-Palestine/ Rhodesian/Zimbabwe situation as much as it is the equivalent of the anglos and the Norman’s becoming a unified English identity (with all the hierarchy and messiness that came from that as well)