Does allocating a worldly reward for completing the memorization of the Quran nullify the reward for the memorization, and what is the advice for achieving continuity in memorization and revision given the envy towards memorizers?

It is permissible to encourage oneself or receive a reward from another person for memorizing the Quran, and this does not contradict sincerity as long as the motive is love for the Quran and a desire for what is with Allah. The Prophet, peace and blessings be upon him, encouraged good deeds with rewards. There is no harm in monetary prizes to motivate students to memorize the Quran, while guiding them to purify their intention for Allah. As for the envy mentioned, if you mean ghibta (wishing to memorize the Quran like them without wishing for the blessing to be removed from them), then it is a commendable matter. However, if you mean wishing for the blessing to be removed from them, then this is not permissible.