It's actually easy to distinguish the different European ethnicities, genetically, since they have the largest, deepest data set.
English were rich and really racist, so they didn't interbreed and quickly sunk into genetic irrelevance, while the Scotts-Irish and Germans took over. Scandis stayed up North, near Canada.
The largest and deepest dataset, absolutely, but raiders and traders blurred those boundaries even before modern transport technologies. Y-chromosomes in eastern England: at least 50% are indistinguishable from stay-at-home Germanics on the Continent. And 10% are Norse.
I also have grave suspicions about "didn't interbreed" in the context of pre-20th C British. Didn't intermarry? Perhaps. Didn't interbreed is lol, esp if they had money.
Most English settlers didn't have money, but there were so many of those its hard to believe they didn't leave a larger genetic footprint. This map has about as much English as I'd expect in Columbia, not one of Australia's siblings.
Open to what you say on intra-US migration, but I'd really like to see data.
