Does inner resentment that lasts for seconds, even with self-reflection and recalling Allah's mercy and wisdom, lead to disbelief?

We did not say that internal discontent (تسخط قلبي) is disbelief (kufr) outright. Rather, it has levels, some of which reach disbelief. As for fleeting thoughts of discontent that do not settle in the heart and which one dislikes, they are not held against a person. Instead, one is rewarded for struggling against them and repelling them. They are excused, similar to the whispers of the self (waswasat al-nafs).