What someone need to be able to train constantly for 500 years straight is a not just talent, it is resolve, conviction. Even in our world there are plenty genius but not like they all become big shots when they grow up.
I would be pretty damned disappointed if the author design MC's personality half assedly.
Someone who can keep his conviction to train for 500 years continously is definitely not sane if you think about it and it is just the kind of thing I like. I think MC's life purpose is surpassing his master and become the real strongest man in this world ( until he set his goal on the gods though ? ). He still have several thousand years training ahead
If Sansui live normally I bet he would still be successful in the future