Is my leading the prayers—when the regular Imam is absent—and my recitation of new verses that I haven't recited in prayer before, motivated by a desire to avoid repetition and the whisperings of reciting specific verses or to demonstrate Tajweed rules, considered showing off, despite my intention to memorize the Quran for the sake of Allah? And should I avoid leading the prayers for this reason?

Satan is keen on corrupting a Muslim's worship, and his whispering during prayer is known as Khanzab. To get rid of his whispering, it is recommended to seek refuge in Allah from him and to spit lightly to the left three times, as 'Uthman ibn Abi al-'As did by the order of the Prophet, peace and blessings be upon him. Satan is an enemy who wants to deter a Muslim from prayer, leading prayer, and memorizing the Qur'an, so one must be wary of him and oppose him.