I went in and pulled the cassette off, took a file to the notchiness on the hub splines, and then the cassette went back on a lot easier. But the end result was the same. THEN I pulled it again and tried to sand down the plastic spacer, I got cog+spacer down from like 9.0mm to 8.5mm, but that still didn't make any difference.

I am convinced at this point that this Sunrace cassette is an example of you get what you pay for. The cogs and built in spacers are not manufactured precisely enough to get accurate shifting.

So now I'm going to settle in and live with it for at least a year before I 'earn' the right to spend more money on a higher quality Shimano or Sram cassette.