The recorded Native American population was at its lowest (around a quarter of a million, not including 25,000 or more Alaska peoples) in 1890-1920, though comparison by age with 1930 suggests that at least the 1920 return significantly understated numbers, despite the ravages of the deadly post-WW1 influenza pandemic: it's likely that the true low occurred in 1900 (Census) or 1890 (returns of the Commissioner of Indian Affairs).

The "closing of the frontier" - the establishment of US settlement from coast to coast - is conventionally dated to 1890, which is also the year of the notorious Wounded Knee massacre, the last such episode on its scale. The last great battles occurred in the 1870s, but local armed conflicts would persist on a smaller scale into the 1920s.

So there's really no precise date. But 1890 is perhaps the strongest contender for the ending of the classic phase of conquest and depopulation. Native Americans might counter that subjugation and disproportionate loss of life continued far later.