Are the mentioned etiquettes for completing the Quran, which include saying "Allahu Akbar" from Surat Ad-Duha, finishing the recitation at the beginning of the day or night while fasting, attending the gathering, starting a new recitation, and dedicating the completion to the Prophet and the Muslims, correct?

The takbir (pronouncement of "Allahu Akbar") in the final surahs is a sunnah (definition) according to Al-Bazzi from Ibn Kathir. It is also prescribed to combine it with tahlil (pronouncement of "La ilaha illa Allah") and hamdalah (pronouncement of "Alhamdulillah"). It has been the custom of the people of Makkah to regularly observe it during Tarawih prayers. Furthermore, supplicating at the completion of the Qur'an is prescribed, as it is authentically reported from Anas. As for completing the Qur'an at the beginning of the day or night, there is no harm in it, and the narrated report from Mujahid does not establish a legal ruling. Scholars have differed regarding dedicating the reward of reciting the Qur'an to the Prophet, peace and blessings be upon him.