How can one ensure having acquired a sufficient amount of the obligatory Sharia knowledge (especially in Creed, Fiqh, and Language) so that one can then focus on learning and contemplating the Holy Quran?

Combining the betterment of this world and the Hereafter is possible and مطلوب (desirable/required), and seeking reward (ihtisab) in the study of civil and human sciences transforms permissible acts into worship and obedience. To ascertain the necessary amount of Islamic knowledge and the method of acquiring it, one can refer to fatwas 63362 and 99857. Among the beneficial books for those who are not full-time students of religious knowledge are "Mukhtasar Al-Fiqh Al-Islami fi Daw' Al-Qur'an wal-Sunnah (definition)" (Abridged Islamic Jurisprudence in the Light of the Qur'an and Sunnah) by Shaykh Muhammad Al-Tuwaijri, and "Al-Targhib wal-Tarhib" (Encouragement and Discouragement) by Al-Hafiz Al-Mundhiri, or "Sahih (definition) Al-Targhib wal-Tarhib" by Shaykh Al-Albani. One must seek Allah's help, genuinely rely on Him, and supplicate to attain goodness.