Is it recommended to say "Bismillāh Ar-Raḥmān Ar-Raḥīm" (In the name of Allah, the Most Gracious, the Most Merciful) in the middle of a Sūrah, in accordance with the ḥadīth: "Every matter of importance that does not begin with 'Bismillāh Ar-Raḥmān Ar-Raḥīm' is cut off or incomplete," or is it not legislated?

The correct view is that the Basmalah is not recommended when starting a recitation from the middle of a surah, based on Allah's saying: "So when you recite the Qur'an, seek refuge with Allah from Shaytan, the expelled." This specifies the generality of the saying: "Every important matter that does not begin with Bismillah is truncated."