While not for everyone to do or make, but those who are serious about leather work.
I made a sander for doing just this thing when I was doing heavy hides and pelts such as buffalo, elk and beavers, etc.
Got some would and glued a block of hard wood together about 15 inches or so, had a friend lathe into a sphere with flat sides, then made me a table for it to fit on with a 1 horse power motor, cut a hole in the center figured out how to mount the sphere in it about half way out the surface, ran it off 110, wet it down with some Elmer's glue and place some fine sand on it and turned it around by hand to get an even coating all over it.
Let it dry a couple of days, then turn that baby on, it was as expected a bit fast so I had to readjust the pulley speeds, like one would do a drill press with belts, was quite easy. I could not stop the rotation on that sphere all it did would take and make holes in the hide if you did not move the hide around,
When my hides were just about dry and almost ready to break, I would sand the flesh side down with this until I felt the thickness what I wanted to, it would sand that leather off just like a grinder on metal, worked slick as a whistle and seeing as the flesh side was always sanded looking any way it made no difference to the finish look, it also made those large and heavy pelts much easier to break.
When the grit was breaking off I took a large file and scrapped it clean when I turn it on then just replaced it out of a 50lb bag I had bought, Only thin is one should set up a vacuum cleaner to the underneath side of the sand sphere as it gets quite a large amount of leather at the base or on the floor in a hurry, when doing elk hides or buffalo.
I think I had about 100 dollars into it including the motor was quite easy to build, took about two days after work after my friend had turn the sphere
Other wise you can use an auto body hand sander, using a knife will make irregular cuts and our subject to nicks as well as you need a wide surface area to cut at once instead of a bunch of small slices.