We all pretty much start from scratch... You're getting there !
Once you figure out how to flow and move the metal, you can get it flatter and smoother and blend in the highs and lows. It takes time but worth the effort to get it as smooth as you reasonably can to minimize the thickness of the filler/glaze which can alter the shape of the surface. Keep in mind that low spots can be filled but highs will necessitate a heavier layer and will raise the surface around it and I try to never have any filler thicker than maybe 1/16".
With a guide coat of primer ( can be just a dusting) , and a flexible sanding block to follow the curves, you can keep working on the small dents and eventually bring the surface back to smooth.
The hard part is deciding when it's good enough because what feels or looks smooth at this stage will become something else when it's all one color and shiny with new paint.
As for grinding...On your tail and the inner curve at the rear of the tunnel, for instance, to fair out and blend in the high welds, you will have to apply a thick layer of filler which will cover the seam strip and you'll lose that detail unless you create it on the surface with filler. which means you'd have to fair that out along the strip and it just builds from there.
I like to use the edge of a cutting disk on the Dremel to carefully sculpt and detail those kind of areas and get the metal shaped right so the glaze only smooths it out and I am not creating too much detail with it. Make sense ?
This is not very dramatic but here is the SS leg shield when I started and then almost ready for filler. The edge had been bashed over and then roughly straightened out. This was typical on this bike. They had banged the dents out crudely but left them so they'd be below the surface but then used no filler under the crappy paint job.
Here is also a dent in the right cowl where you can clearly see that they just 'picked' out the center of the circular dent from the back and then hit those small dents back down. You can see the original dent and the raised ring around it from just popping the center back up. This certainly could be filled but there would be a general high bump on the panel.
I like to use a sharpie and denote the low spots or areas by their edges and the high spots with hash marks and work them up and down to where they blend as closely as I can get them.