I don't know how that is in the rest of the world, but being an American is like bringing a dish to a pot luck dinner: You have a little bit of everything. So it was that I've seen a naturalized Sikh who wore traditional clothing, though his son or in-law didn't (we discussed growing tomatoes). Maybe it was less of an adjustment for a former Lithuanian I knew, but we never got into that. I know a naturalized citizen from Portugal who has kept part of her culture, and that's typical. The assimilation part concerns views of government, of liberties, and yes, history, too, and a desire to be part of society and not apart from it.
Now, since you brought it up, there is a strange notion that's cropped up in the last few decades that treats culture as though it's genetic. Had heard of it, but didn't encounter it until the eldest offspring was required to write an essay "What is your culture?" I assume the expectation was that this would connect to where families had come from. Except culture changes. I don't make a living by hitching a mule to a plow like my grandfather and his father and the ones before him did. I don't wear the same clothes. At Christmas we put up a Christmas Tree, something that's surprisingly recent as such things go. My wife has some Indian ancestry, and I've been asked by several Indians if I belong to a particular Indian nation, so they may see something there, So our eldest wrote that we had been in the US so long we weren't sure where all our ancestors came from, and our culture what that of the region where we lived. And really, unless someone deliberately decides to live apart from the culture, that's the case for everyone. As it turns out, our ancestors were here before there was a US. But my culture is radically different from there's.
You wrote that the Yankees didn't adopt the culture of the Massachuset or the Wampanoac, but those who first settled did adopt part of it, They had to in order to survive. Not just agriculture. Buckskins were essential wear on the frontier because brush would shred the cloth of the time. Tanning was usually done like the Indians did it - brain tanning and smoking. Vegetable tanning was also done, but seems that it didn't yield the same results.
In that same vein, until the 20th Century my family grew a bean in corn, planted when the corn was "laid by," that would grow up the dying stalks after the corn had been harvested and served as fodder when cows were turned on it. That sounds suspiciously like a partial adaptation of the Three Sisters method. I also wish I talked to my father more about how they tanned their own leather, but it was a job he despised growing up, so don't know how much he would have told me. But the gist of it sounds for all the world like it was relying on the natural tannins in creek water, and I don't know if that's something they figured out or maybe if it was something indigenous.