Here's what the comparison you're raising actually contains, if you follow it honestly.
Brazil imported approximately 4.9 million enslaved Africans over the course of the slave trade, more than any other country in the Americas.
The mortality rate was so high that the population did not sustain itself, requiring continuous importation until Brazil became the last country in the Western Hemisphere to abolish slavery, in 1888.
The United States imported approximately 400,000 enslaved Africans.
By 1860, the enslaved population had grown to 4 million.
Do the arithmetic.
400,000 people became 4 million through forced reproduction across generations, in a country that had made it illegal to import more.
American slaveholders did not work enslaved people to death as fast as Brazilian sugar planters.
They bred them instead.
They separated families to sell children at profit.
They used enslaved women as reproductive assets.
They tracked bloodlines and priced people by their fertility and the health of their offspring.
They created what historians call the "domestic slave trade", an internal market in human beings, powered by forced reproduction, that was generating more capital by 1860 than all American manufacturing combined.
You raised this comparison as though it softened something.
It doesn't soften anything.
It adds a dimension of horror that the simple mortality rate comparison was designed, by you, to avoid.
Have you compared the rate at which slaves died due to the conditions of their enslavement in the US versus for instance, the Sugar plantations of the Caribbean and Brazil?