Nothing you just said means he's invincible.

The entire point of him is that he's literally invincible. That's half the joke.

Because nobody in OPM can stand a chance against him. Saitama was not bitten by a radioactive Mary Sue, he just trained himself to the point that he's the strongest character in his own series. Doesn't mean he'd instantly win fights against stronger characters from stronger universes in just one punch.

The creator has stated that a single full strength punch by Saitama would contain about half the energy of the big bang.

When? Genuinely never heard of this one before. And also, this implies Saitama's maximum strength is just as strong as half of the Big Bang. Strong, yes, but not invincible.

The whole deal with Saitama is that he's so reality breaking and beyond powerful that all of his problems are entirely mundane and small despite the fact that he occasionally fights beings that are universe threatening Gods.

He has not really fought any outright "universe-threatening god" yet, his strongest foe so far was just God Garou.