Is not mastering all that was previously memorized from the Qur’an considered a sin, despite forgetting it and not knowing the punishment for that at the time of memorization, and being ignorant of the obligation to regularly review the Qur’an because it is the word of Allah?

Forgetting the Quran due to neglect is a sin, but forgetting it without any shortcoming is not blameworthy. If the forgetting is due to neglect, then you must seek forgiveness and strive to review in order to restore what you have memorized. If you are unable to recall some of it after striving, then there is no sin upon you. Your repentance will benefit you, and "the one who repents from sin is like one who has no sin."