Historically land was named after the ethnicity that occupied the land and controlled the government. China, Korea, India, Japan, Germany, France, Denmark, Sweden, Turkey, and Greece are all examples of nations named for the dominant ethnicity that controlled the land.

The USA and Canada are very different. Though Quebec was once called "New France" and "New England" was named after the settlers from England. So, nations like the USA and even Canada had to define themselves differently than by ethnicity and so created "Constitutions" that outlined their values... later other countries adopted Constitutions but that was long after they had been defined. Even today many African and Middle Eastern countries have raging conflicts because the people don't identify with the nation because they are of a different ethnicity.

So, when the Founding Fathers drew up our "social contract" which was pretty unique (most nations didn't just burst on the scene like we did, they evolved over centuries), they had to find a way to knit 13 distinct colonies who had just gained their freedom into something closer to a singular nation and used values and rights to do so and then overlayed those values and rights with a government structure.

Quite a unique thing at the time, so much so that Franklin commented that they had created a "Republic" and then cautioned that it would be a challenge to keep it.

Regarding, your comments about slaves, Native Americans, and women not having equal rights. That is true, while the country was founded on the aspirational belief that all humans were created equal (a concept that arose out of the Reformation and went on to become what we call "classical liberalism" today. It would be wrong to suggest that people of the time fully bought into that "vision". In fact, when MLK gave his dream speech, he was not saying something "new", but simply reminding Americans that from the beginning we were supposed to be a nation where all humans are created equal and that we had not yet achieved that goal.

So, in the beginning identity politics or tribalism whichever label you prefer, resulted in whites believing them superior to other races and men believing themselves superior to women. And even today, separatism when expressed by the KKK, Nation of Islam, Amish, Orthodox Judaism, radical Islam, etc. is alive and well. In the 60s, one of the biggest advocates of separatism was Malcolm X. Rather than believing in MLK's dream of the "melting pot" where we would begin with legal integration through laws to end institutional and systemic racism, followed by social integration and ultimately biological integration, Malcolm X promoted the idea that different tribes would live side by side and peacefully contend for power... but if peace did not work, then violence would be necessary. His statement by any means necessary highlights that violence while not preferred would ultimately be called upon to gain power. The flaw is such an ideology inevitably leaves to violence. You would think after Jim Crow the nation would understand that separate but equal is a lie.

So, you are right, the struggle to realize the original vision of society where individuals enjoy individual identities, tolerate differences, and over generations integrate first "legally", then socially, and eventually "biologically" to form a "new nation" continues to struggle against those who see benefits in a separate identity even if such separatism leads to conflict.

As to which ethnicity defines Americans... none yet. But if you believe the evolutionary biologists, America which was founded on "shared values" will transform into a new "people" through biological integration... that group will be genetically "Hispanic", but with a greater amount of Asian, Northern European, and African DNA than other Hispanic ethnicities that are largely southern European and Native American. This could take hundreds of years and as you point out just like in other countries, there will remain small minorities that have opted not to integrate, but if we preserve our core values, they will be able to coexist with the dominant "New Americans". My family is well on its way down this path with my wife being Asian, and five of seven grandchildren being African American. Four or five generations from now, I suspect our DNA results will comeback "fully American".