I can't see that "Amazing Grace" has anything to do with slavery.
John Newton left the sea (and slave trade) in 1755. He was ordained minister in April of 1764 and made curate of Olney. William Cowper moved there in October, 1767 and together (but mostly Newton) they wrote the songs in 'Olney Hymns', 1779.
I haven't seen this work, but understand that "Amazing Grace" first appeared there, and without music or tune direction.

Newton left Olney in 1779 to become rector at St. Mary Woolnoth, London, where he died Dec 3. 31, 1807.