You could work w/ some early changes of papal bulls. Dum Diversas (1452) and Romanus Pontifex (1455), both by Nicholas V, authorized the enslavement of muslims and pagans to protect christendom
w/o said bulls maybe there can be some butterflies at the Valladolid debate, say Las Casas doesn't advocate for african slavery to substitute native one, say maybe Sublimis Deus (1537) condemns the enslavement also of pagan natives, say this impacts CV's new laws enough that PII/PIII feel like using slavery as a propaganda tool against the dutch and other protestants
Could lead to spain-portugal slowly taking tougher stances on mistreatment of slaves and slavery of non-muslims over the course of the XVII century
catholic arguments against slavery (mainly by jesuits) appear and start circulating more and more and during the thirty years war, at the height of catholic v. protestant tensions, some german state, maybe austria, pressed between the muslim and protestant worlds, bans it out of spite. So, later, Leopold I can use it as an additional justification in fighting the turks
Over time slavery becomes associated w/ protestantism in popular conscience (even though many catholics still practice it prbbly), which could result in say the edict of fontainebleau citing it as an additional reason for the expulsion and in catholic countries banning it in their metropoles
Prbbly the whole process is accompanied by worsening conditions of serfs, similarly to what happened in russia