Is there something specific I must do for Allah to accept my repentance, or is it sufficient to simply return to regular prayer? And how can I make up for the missed prayers and Jumu'ah prayers throughout my period of illness? And is that considered leaving the religion?

Abandoning prayer is a grave sin and a serious transgression. The majority of scholars hold that one who abandons prayer does not exit the fold of Islam, but they commit one of the greatest sins. The position of the majority is the obligation to make up missed prayers, and this is more cautious. Making up prayers should be done according to one's ability, in a way that does not harm the body or livelihood. Some may consider it sufficient to make up two days' worth of prayers with each day [of current prayers], and this is the view of the Malikis. Missed Friday prayers should be made up as Dhuhr. It is obligatory to repent to Allah and be diligent in performing prayers on time, especially Friday prayer and in congregation. Your diligence in worship and drawing closer to Allah is a means for the removal of harm.