Is grumbling about one's fate, even if not fully conscious, considered an act of disbelief (kufr)? And what are the consequences of such an act?

Objecting to and being discontent with divine decrees is a reprehensible and forbidden act, but it does not reach the level of disbelief (kufr). A Muslim must repent from this and accustom himself to patience and contentment with Allah's decree, knowing that Allah possesses profound wisdom in all His judgments. Overcoming blameworthy anger is achieved by striving in supplication (du'a), remembrance (dhikr), and self-struggle (mujahadat al-nafs).