Is it permissible for someone praying alone to vocalize the prayers in Maghrib, Isha, and Fajr, and is saying: "Our Lord, to You be all praise and thanks" after bowing an innovation?

Reciting aloud during the aforementioned times is permissible by scholarly consensus. It is a Sunnah for the Imam according to the three Imams (Malik, Shafi'i, and Ahmad). The Hanafis consider it obligatory for the Imam in audible prayers. It is also Sunnah for someone praying alone according to the Malikis and Shafi'is (and is one narration from Ahmad). However, adding the word "wal-shukr" (and thanks) after saying "Rabbana wa lakal-hamd" (Our Lord, and to You be all praise) is not appropriate, as it has not been authentically reported from the Prophet (peace be upon him).