>minus the epstein drive which is basically magic
It's canonically a fusion rocket, isn't it? So beyond today's means, but hardly "magic".
And most of the story wouldn't even change if it took longer to go places. The only difference would be that they couldn't show normal gravity inside a spaceship and attribute it to acceleration. It's a slight distortion that saves them money on special effects.
(This is not merely a detail, it's one of the core plot points of The Expanse, as tolerance to gravity is what makes Martians and Belters almost a different species from Earthers; the former can not live on Earth, and so they don't care much about it.)
I don't actually recall what the in universe explanation of it was outside of it running off water as its fuel. Either way its specific impulse is crazy.