Flattening the sole was very difficult. Stainless steel hates heat and moves as it heats up. If you are using power to sand/grind it, it gets really hot and I have nasty burn to prove it (stainless is a poor conductor of heat, compared to “regular” steel, thus the heat isn’t dissipated throughout the steel and remains localized). So, if you use power without coolant, you are chasing a moving target. Took me a while to figure this out as I used glued sand paper, files, and a 12” disk sander and didn’t seem to be making any progress. Eventually, I got to the point where hand sanding on glued paper got me close. But not very flat as the paper left hills and valleys. From there, I used a large diamond hone to get to the point where I didn’t care anymore. You’ll notice the “wave” behind the mouth, like that on a Japanese plane. I’d like to say that was intentional, but no, that was caused by the sole being banana shaped after brazing. But it doesn’t hurt and may even help.
This is idle speculation on my part, but I think having the sole be very flat at the front (at the mouth) is important when planing against the grain as the chip is held down and has to break before it gets a chance to lift and tear. Planing with the grain, who cares? it isn’t going to lift.
One fun thing to do is attempt to pick up a piece of wood with the sole, this gives you an idea of how flat the sole is. Take a small, very smooth piece of wood (I use 1/4” hard board, which has a “polished” surface), press the sole down on it, “wring” (twist) it and lift the plane. The wood should lift off an inch or two before it drops away.