I found this website to be REALLY helpful when diving down the rabbit hole: http://www.coinimaging.com/
The author specializes in photographing coins, and has tested a plethora of different types of lenses for that purpose but still quite applicable to digitizing film. You can even see the results of some microscope lenses if you’re considering one of those!
Another good resource is https://www.imaging-resource.com/lenses/reviews/. Good sharpness and flat-field analysis for most modern lenses. Helped me decide to go for my current macro lens.
I was using a Schneider Componon 50 2.8 with the Canon slide duplicator and Bellows FL for 135, and an EL Nikkor 135mm 5.6 with a custom 3D printed holder for 6x7. You’ve gotta remember to stop those lenses down after having them wide open for setting focus, otherwise you’ll definitely notice soft edges. I got good results with both, especially with the Nikkor on some studio-shot portraits with my RB67 and 90mm KL lens. With the Schneider, I mostly scanned some old slides that were shot with a camera with a less than sharp lens, but the grain came out looking pretty good!
Recently, I decided that the whole set-up can be GREATLY simplified if I just bit the bullet and bought a modern macro lens. Plus, I could use if for more that just camera-scanning, like product shots or flowers and stuff. So I picked up a Tamron 90mm macro for $200, haven’t regretted it at all. Super sharp all the way to the corners.
As for your Nikon 60mm macro; I’m not too familiar with it’s performance, but sounds like you need to stop down the aperture to find it’s best corner performance without sacrificing over-all image sharpness. You’ll have to find that sweet spot with experimentation!