Ok, spent a little more time, got a variety package of auto body sandpaper, and was mindful of the "don't ruin it in the quest for perfection" advice. Turns out that point became obvious - I could tell that I wasn't improving it anymore, so I did a final block sanding up to 600 grit and tested it out. Shuddering is now barely noticeable and I'm sure it will improve over time by itself. Calling it a day. Thanks again to all.
Follow-up: put a couple hundred miles on the fix, is now braking with no shudder. I'll keep an eye on the rim to look for any problems, but so far, the repair is a success.
Nothing succeeds like success. Odds are that this will stay OK until the rim dies from other causes.
Originally Posted by rsacilotto