Should one help one's parents prepare a late dinner, such that it affects waking up for Fajr prayer, or should one sleep early to wake up for the prayer, leaving the parents to prepare dinner themselves?

Preparing dinner for your parents is one of the greatest acts of filial piety and a cause for the alleviation of distress. You can combine preparing dinner with waking up for Fajr by taking a nap during the day, going to bed early at night, and using an alarm or asking someone to wake you up. If you cannot combine them, then prioritize filial piety and prepare dinner for them, then take the necessary steps to wake up for Fajr. If sleep overwhelms you after taking these measures, then the time of prayer is the time you wake up, and there is no sin upon you, because the Pen is lifted from the sleeper.