Does the prayer become void if the worshiper focuses only on the word "Akbar" from the Takbirat al-Ihram without focusing on the rest of it?

When you begin prayer, after fulfilling its conditions, it is sufficient for you to intend to pray and then say: "Allahu Akbar," because the Prophet, peace and blessings be upon him, said to the one who prayed incorrectly: "When you stand for prayer, say 'Allahu Akbar,' then recite what is easy for you from the Qur'an." What you feel, a lack of focus during the opening Takbir, may be a whisper (waswas), and you should ignore it; your prayer is valid.