This article is all well and good but realistically, in 2020, if you were a teacher at any grade level who taught about slavery or colonialism with any nuance whatsoever your career would be promptly executed by internet firing squad.
I found that the moral ambiguity only ratcheted up over time. In 3rd grade it was peaceful pilgrims co-existing with the natives. In 9th grade it was the trail of tears and Vietnam. In college it was "the sons of liberty were terrorists and the noble savage trope is a myth".
Unless you're black or some other 'colonized' skin color, of course.