To what extent is the division of impurity into "major ritual impurity" (encompassing shirk, kufr, and major sins) and "minor ritual impurity" (encompassing ritual impurity and minor sins) sound, and did any of the Salaf articulate this?

Purity and impurity are divided into two categories: sensory and spiritual. Sensory purity is purity from ritual impurity (hadath) and physical impurity (khabath), and sensory impurity is that which religious law has deemed impure; this is the focus of the jurists. As for spiritual purity, it is the purity of the believer from polytheism (shirk) and disbelief (kufr). Spiritual impurity is the impurity of disbelief, wickedness (fusooq), and disobedience (isyan). There are religious texts that indicate this, among them His (Allah's) saying: "and purified you" (وَطَهَّرَكِ), meaning purified your religion; and His saying: "you purify them" (تُطَهِّرُهُمْ), meaning from the defilement of their sins; and His saying: "and to purify you with a thorough purification" (وَيُطَهِّرَكُمْ تَطْهِيرًا), meaning spiritual purity; and His saying about the people of Lot: "Indeed, they are a people who purify themselves" (إِنَّهُمْ أُنَاسٌ يَتَطَهَّرُونَ), meaning from sins and indecencies; and His saying: "Indeed, the polytheists are impure" (إِنَّمَا الْمُشْرِكُونَ نَجَسٌ), which is a spiritual impurity. Major polytheism is a severe impurity, and minor polytheism is a light impurity.