Is saying, "O Allah, I repent to You for my ingratitude for Your blessings," considered disbelief (kufr) if it stems from an inner belief—without being uttered—that the Qur'an is not a cure but rather a means, and that the cure is in Allah's hands, knowing that the Qur'an described itself as a cure?

The phrase mentioned is not considered disbelief, and what goes on in a person's mind without being uttered or acted upon is not held against them. The Qur'an is a healing and a mercy for the sincere believers, but it only increases the wrongdoers in loss, as it stands as a proof against them. Ibn al-Qayyim explained that the Qur'an is a complete cure for all ailments of the heart, body, worldly life, and the Hereafter, for whoever seeks healing from it sincerely, with faith, and firm conviction.