If you want sharper overall images, consider a lens with a larger image circle such as a Tilt-Shift lens. Distortion will be low, and sharper corners and less viginetting.
You might also adapt a MF lens, but most low cost ones will not match a 35mm lens.
All Macro lenses are very good. I'd go for contrast and color rather than worry about a tiny amount of difference in distortion. Realistically, you have to spend a lot of $$ to get much improvement over your existing lens. Money might well be better spent on lighting.
I've got the 24 TS-E, and it's awesome. It's also a bit short for this sort of thing, though I might wind up using it for an oversized canvas. I'm holding off on additional TS-E purchases until after Canon updates them.
Can you offer any suggestions for specific medium format lenses to consider?
Contrast and color are important to a degree, yes, but differences are almost entirely eliminated with a good ICC profile. Distortion, at least for me, is much more challenging to correct for. And even slight amounts of distortion can be jarring in artwork with lots of straight lines.
Lighting I've got covered...four Einsteins. When I configure my studio for copy work, there's perfectly flat, glare-free illumination on the artwork.