Is it a sound approach, or does it involve injustice, to be content with educating girls only up to the preparatory stage, then requiring them to stay at home to memorize the Qur’an, Sunnah, and Islamic ethics, while providing means for home-based education?

This approach is good and may even be obligatory if corruption becomes widespread and there is fear of fitna (strife/temptation). There is no contradiction between women learning the Islamic sciences and pursuing permissible academic education, which may sometimes be a collective obligation (fard kifayah). Academic education is not prohibited unless it involves something forbidden, such as uncovering men's 'awrat (private parts) or an external matter like intermixing (ikhtilat) and forced removal of the hijab. If it is free from such things, it is Islamically permissible, provided the woman adheres to the hijab and lowers her gaze. Girls can be enrolled in Al-Azhar education because it is not mixed.