To ease your mind a bit, you're absolutely correct to assert that it's not as simple as x and y, and that the structure of race that was used to justify transatlantic slavery wasn't built in a single stroke. To really grasp how race and racism become concrete, well defined concepts and tools of oppression, it's much more fruitful to consider how terms like savage and barbarian were first used in almost all colonial era writings, and how coupled with enslaving a group those connotations associated with west Africans naturally evolved, while at times being deliberately modified to justify or apply moral defenses. The first time we have record of west Africans being taken in large groups as slaves as sort of a catalyst for transatlantic slavery is estimated at 130,000 west Africans in the period between 1441 and 1512 and in the Americas 300,000 slaves between 1520 and 1619. To point a single finger in all of this is letting all those who upheld and profited off of the institution of transatlantic slavery off the hook. That being said, there is some merit to the claim.

At the time that Zurara was writing about the Africans he came in contact with, race was not yet a distinct concept, but he does exist as the first written record of someone separating peoples characteristics based on skin colour. Those first distinctions were generally alongside religion and implied inferiority, and the popularization wouldn't have occured without the Catholic Church. The language of the time would define Africans as barbarians, or Savage's, and heavily relies on religious undertones. The Iberians set up a heirarchy that would resemble racism, and over time, all across Europe some derivative of the word negro would become synonymous with slave. There was a lot of religious undertones to the first campaigns in slavery. James H Sweet writes in his 2003 paper:

"The first transnational, institutional endorsement of African slavery occurred in 1452 when Pope Nicholas V issued the bull, Dum Diversas, which granted King Afonso V of Portugal the right to reduce to “perpetual slavery” all “Saracens and pagans and other infidels and enemies of Christ” in West Africa. In 1454, the Pope followed up Dum Diversas with Romanus Pontifex, which granted Portugal the more specific right to conquer and enslave all peoples south of Cape Bojador. Taken together, these papal bulls did far more than grant exclusive rights to the Portuguese; they signaled to the rest of Christian Europe that the enslavement of sub-Saharan Africans was acceptable and encouraged. Later conflicts over the rights of conquest and trade in Africa did nothing to change these understandings. Portuguese, Spaniards, and other Europeans contested the African trade, but these were little more than internecine economic and political squabbles. From a social, cultural, and philosophical perspective, all were Europeans, and all underscored their rights to enslave Africans on the grounds that theirs were “civilizing” missions."

In Romanus pontifex, the pope writes:

We [therefore] weighing all … with due meditation, … grant among other things free and ample faculty to the aforesaid King Afonso—to invade, search out, capture, vanquish, and subdue all Saracens and pagans … and other enemies of Christ … and to reduce their persons to perpetual slavery …. Thence also many Guineamen and other negroes, [already] taken by force, and some by barter …, or by other lawful contract of purchase, have been sent to [Portugal]. A large number of these have been converted to the Catholic faith, and it is hoped, by the help of divine mercy, that if such progress be continued with them, either those peoples will be converted to the faith or at least the souls of many of them will be gained for Christ. (English translation of Romanus Pontifex by Pope Nicholas V, as published in European Treaties bearing on the History of the United States and its Dependencies to 1648)

To put all of this together a bit better, I think it's fair to say that Zurara is the first racist and an important figure in creating racial hierarchy in the 15th century and is the first written record of justifying the slave trade, but I think there is evidence to suggest that he is more of a scapegoat to a larger institution of perpetual racial slavery that had already begun involving the royalty and although Zurara supported his country in their pursuits and helped promote and instill the notions of racial inferiority, he wasn't capable of organizing everything involved in the first slave auctions.

Kendi writes in his book stamped on a chapter titled 'The story of the world's first racist':

"He wrote the story, a biography of the life and slave trading of Prince Henry. Zurara was an obedient com- mander in Prince Henry’s Military Order of Christ and would eventually complete his book, which would become the first defense of African slave trading. It was called The Chronicle of the Discovery and Conquest of Guinea. In it, Zurara bragged about the Portuguese being early in bringing enslaved Africans from the Western Sahara Cape, and spoke about owning humans as if they were exclusive pairs of sneakers. Again, this was common. But he upped the brag by also explaining what made Portu- gal different from their European neighbors in terms of slave trading. The Portuguese now saw enslaving people as missionary work. A mission from God to help civilize and Christianize the African “savages.” At least that’s what Zurara claimed.

Edit: Sorry, I forgot to answer more directly your last question. Kendi spent time digging through all written records surrounding early colonial slavery, and the first time he is able to distinguish racist phrasing is within Zurara's biography 'The Chronicle of the Discovery and Conquest of Guinea'

Sources/relevant information:

Spanish and Portuguese Influences on Racial Slavery in British North America, 1492-1619 James H. Sweet, Florida International University

Stamped from the Beginning The Definitive History of Racist Ideas in America Ibrahim X Kendi 2016

I very much hope you will get a full answer from one of our experts, I would myself like to know this and I am aware there has been a lot of research on the subject recently. However while you are waiting I can repost a list of earlier threads on the history of racism below:

• How would an Egyptian in Alexandria at the turn of the millennium have perceived race and skin tone? by u/cleopatra_philopater
• Did the Prophet Muhammad (pbuh) ever say this? by a now-deleted user
• Why are "Moors" often depicted as black in European art? by u/sunagainstgold
• There's been a lot of talk here about how medieval Europe was more racially diverse than we think. Would it be out of the question to see a white person in 1000s-1300s Sub-Saharan Africa? by u/labarge3
• Europe had normal diplomatic relations with non-white nations before turning explosively racist to justify their actions against all others. What happened to cause this shift? by u/victoryfanfare
• How do contemporary academics who say that race and racism date to the Enlightenment explain what seem to be examples of racial categorization, racial stereotyping and racial prejudice in ancient literature and art? by u/restricteddata and u/PhiloSpo

I hope this may be at least somewhat helpful, thanks for asking such a great question!