Is repeatedly reading the Qur’an without memorizing it, but with the intention of memorizing it later, considered a sin?

Memorizing the Qur’an is not an individual obligation (fard al-ayn) except for Al-Fatihah. The purpose of reciting the Qur’an is to ponder its meanings and act upon it. Since memorization is recitation plus an additional effort, memorization is better than mere reading. We recommend memorizing as much as is easy for you.