Celebrating Khatm Al-Quran (completing the Quran) is not part of the Sunnah. Yet, if done culturally—as a joyful occasion without believing it's a religious act—it is permissible. However, any religious practice, such as connecting the end of the Quran to the beginning, must be based on authentic evidence. Making Du`a’ at the time of completion is supported by reports from the Companions, like Anas ibn Malik.

1- Bid'ah means something that has been introduced into the religion of Allah that has no general or specific basis to support it. 2- There is nothing wrong with praying more than 8 rak’ahs in Taraweeh prayer. For more, see the detailed answer.