How can I maximize sharpness and detail when photographing marble sculptures in a studio?

It sounds like you're doing almost everything right, but there's one detail that caught my attention: Aperture highest the lens offers. I'm assuming that this means that you are stopping the lens all the way down. You shouldn't do that, because the small aperture results in a less sharp image overall due to diffraction. See What is a "diffraction limit"? for more.

Take a look at How do you find out the "sweet spot" of a lens?, and experiment (two stops down from wide open is a general rule of thumb. With sculpture instead of a flat artwork, depth of field is of course an issue, but it appears that the works in question are relatively flat. (And when they aren't, having some distinguishable depth of field blur isn't necessarily a bad thing as you translate from one medium to another.)

You might also try techniques involving super-resolution — take multiple photographs and combine them.

I would also, just as an experiment, take a few shots with live view autofocus, and compare that to your manual-focus results. I don't think it will be better, but it'd be interesting to see.

And finally, you mention that they're well-lit, but it's never going to hurt to increase the light, if you can.