How can one repent and expiate major sins, such as repeatedly swearing falsely upon the Quran, defying God, invoking evil upon one's family, bearing false witness, slandering, backbiting, and gossiping, while feeling remorse and a sincere desire to return to God?

Allah accepts the repentance of a servant, no matter how great their sin, including disbelief (kufr), major sins (kaba'ir), and violations of the rights of other people, as long as they do not reach the death rattle (ghargarah) or the sun does not rise from the west. Repentance from disbelief and sins related to Allah is achieved through sincere repentance (tawbah nasuh). As for sins related to the rights of human beings, they require seeking absolution from them and returning wrongfully taken rights. If seeking absolution is impossible, one hopes that Allah will appease them. It is also obligatory to supplicate and seek forgiveness for those who were backbitten and to mention their good qualities.