Must I repent of apostasy if, at the beginning of my legal accountability, I did not pray or was negligent in prayer, without knowing that abandoning prayer is a major disbelief that expels one from the fold of Islam?

According to the majority of the four Madhhabs, abandoning prayer is not considered disbelief (kufr) unless, according to the Hanbalis, one is summoned to it by the Imam and refuses. Therefore, you are not required to repent from apostasy. You must make up the missed prayers, and their number should be estimated through diligent effort until you are reasonably certain that you have fulfilled your obligation.