Is a lack of enthusiasm for life, coupled with a feeling of futility, contentment with prayer, and not complaining to one's family, considered an act of disbelief (kufr)?

This feeling is not disbelief; rather, it is a misconception. A believer lives to please Allah, and a long life increases their good deeds. Make your concern to draw closer to Allah, seek religious knowledge, engage in da'wah (inviting others to Islam), and strive in your studies. Have a good opinion of Allah and ask Him of His bounty, for in marriage there is an opportunity to cultivate the earth by obeying Allah and raising offspring. A believer is always in a state of good, like rainfall, so there is no need for sadness or a negative outlook.