Two major things that are missing in this discussion:
Omani Empire, and the arab slave trade in the Indian Ocean.
Portuguese slave trade starts already in the 1300 in Africa, with sea and land expeditions from Morocco to the south (guinea).
1519 date is clearly wrong. 1815 date is anglo-centric, but fine.
See the catholic church problems in the 17th, 18th, 19th centuries. Protestantism was the problem for them not slavery.
The policy regarding slaves was, is morally bad, negroes are sons of god too, they deserve to know god (some fringe missionaries preached only to slaves and wanted to build churches for slaves), but since negroes are of lesser race, lazy ...