Is the mentioned method for reviewing the Qur’an correct, and is it permissible to start more than one complete reading (khatmah) of the Qur’an at the same time?

The mentioned method of revision is good, and its effectiveness varies from person to person. What is important is to regularly review the memorization of the Quran so that it is not forgotten. It is also recommended to recite the Quran to a skilled teacher, because the original way of learning the Quran is to receive it from a teacher. This is how the Quran reached us: Gabriel used to recite it to the Prophet (peace be upon him), and the Prophet would recite it to his companions. As for the mentioned method of completing the recitation of the Holy Quran multiple times by repeatedly reading one juz' (part) and then moving on to repeatedly reading the next, it is hoped that you will be credited with this number of complete recitations.