Is it prescribed to say "Bismillah" when beginning recitation from the middle of a surah, and is saying "Great is Allah's truth" after finishing the recitation considered an innovation (bid'ah)?

If the reciter begins from the middle of a Surah (definition), he has the option of either saying the Basmalah or omitting it, as stated by the scholars of recitation. As for saying "صدق الله العظيم" (Allah, the Most High, has spoken the truth) after finishing the recitation of the Qur'an, there is no evidence for it from the Sunnah (definition), and adhering to it is considered an innovation, because restricting dhikr (remembrance of Allah) to a specific time, place, or condition requires evidence. The best course of action upon finishing recitation is simply to be silent, as the Prophet (peace and blessings be upon him) did when he said to Ibn Mas'ud: "Enough for you."