The diamond tools are far too dull to cut leather and would just grab and snag and damage the surface — a drag knife is a far more workable tool for leather, so long as it isn’t used at a depth beyond which it can cut safely/consistently.
While not a decorative cut, I was able to use a drag knife to successfully cut leather (workholding was double-sided tape — profligate, but it works):
Spotted the holes using a 30 degree V endmill and the new drill toolpath, then enlarged them w/ a rotary punch — should have spaced them a bit more widely, would have made stitching easier to get even.