Sanding lines don't really go away until you get up above 1000 grit. The problem with going that high on the grinder is those high grit belts don't last a farting spell. People get around this a couple of ways, such as using a cork belt or a scotchbrite belt. Neither of them really make the lines go away completely, but they do a good job of blending those lines into a satin sheen that is pretty to look at. The next step after that is a buffing wheel.
Knowing the challenges you have with doing a lot of hand work, you may want to consider an alternate finish. A small air compressor and a blast cabinet can be had for not much money, especially used. You don't need boatloads of air to glass bead a little knife or even a big cabinet. Alternately, using carbon steel also opens up possibilities like a forced patina by coating the blade in mustard, etc. Those scratch lines will take on a textured look that a lot of people really like.
The popularity of tactical knives and bushcraft knives has opened people's eyes to finishes that aren't supposed to look refined. There are a lot of people out there that prefer the rugged look nowadays.