Did I commit a major sin by blaspheming the Divine Being in my youth out of ignorance, and does repentance from it require me to re-utter the Two Testimonies and perform a ritual bath?

As long as you adhere to Allah's commands and abstain from what He has forbidden, what you did in your youth is forgiven. This is because if an apostate utters the two declarations of faith as a penitent, he is considered to have returned to the religion of Islam. The Prophet, peace and blessings be upon him, said: "I have been commanded to fight the people until they testify that there is no god but Allah and that Muhammad is the Messenger of Allah… If they do that, their blood and their property are safe from me, except by the right of Islam, and their reckoning is with Allah, the Almighty and Exalted." Your commitment today includes uprightness, which is sincere repentance. So, let your heart be at ease and do not pay attention to intrusive thoughts and whispers.