" You may idealize the religious tolerance of their polytheism, but what that matter if it isn't actually serving the spiritual needs of the people?"

Rome in the end was a decadent, but brutal empire full of slaves. And to a slave christian salvation sounds great.

But before there was a empire with emperors taking up the idea of becoming gods themself, there was a republic. And also after it became an empire, they did not have a institution like the inquisition shaping thought and banning heresy baked into their system.

This is the fundamental difference that I see.

In medieval times being expelled from the church was pretty much a death sentence. In roman and greek times for most of its existence not really.

The demise of the Roman Republic was an inevitability. It could have been Sulla rather than Caesar, and if not Caesar it could have been another, but one way or the other the situation was fundamentally unstable and the public was deeply discontent. Would be reformers like the Gracchi were finding enormous popular traction only to get assassinated.

Also, the Roman Republic were prolific slavers too. I say this because you speak of the Empire and slavery but then go into a "But the Republic.." This isn't Star Wars, you can't divide it into good guys and bad guys, the Republic and Empire were both imperial sons of bitches who conquered territory and took civilians as slaves. The demand for reform that would eventually motivate mass conversions to Christianity was already well established before Caesar was even born.