What is the context of revelation for this āyah?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. A man named Marthad ibn Abī Marthad was sent by the Prophet ﷺ to Makkah to rescue some Muslim prisoners there. When he arrived, a woman named ʿAnāq, who had been his mistress in the pre-Islamic era, heard of his presence. When he embraced Islam, he turned away from her. She came to him and said, "Woe to you, Marthad! Won't you come be alone with me?" He replied, "Islam has come between me and you, but if you wish, I will marry you. When I return to the Messenger of Allah ﷺ, I will seek his permission for that." She said, "Would you reject me?" and cried out for help against him. They beat him severely but then let him go. When he had accomplished his task in Makkah, he returned to the Prophet ﷺ and asked him, "Is it lawful for me to marry her?" Thereupon, this verse was revealed. [Ibn ʿAbbās]
2. ʿAbdullāh ibn Rawāḥah had a female slave. One day, he became angry with her and struck her. Feeling remorseful, he went to the Prophet ﷺ and informed him of what had happened. The Prophet ﷺ asked him, "What is she like, O ʿAbdullāh?" He replied, "O Messenger of Allah, she fasts, prays, performs ablution properly, and bears witness that there is no deity but Allah and that you are the Messenger of Allah." The Prophet ﷺ said, "O ʿAbdullāh, she is a believer." ʿAbdullāh swore, "By the One who sent you with the truth, I will set her free and marry her!" He did so. Some Muslims criticized him, saying, "He married a slave woman," while they themselves desired to marry polytheistic women due to their lineage. Thereupon, this verse was revealed. [Al-Suddī]

Some commentators have mentioned that the story of ʿAnāq and Abū Marthad was the reason for the revelation of part of this ayah: "And do not marry polytheistic women until they believe", and the story of Ibn Rawāḥah was the reason for this part: "And a believing slave woman is better than a polytheistic [woman]".