LKA 

Since it was the last thing changed, I swapped out the 11-30T DA 9100 cassette with the previously good-shifting Ultegra r8000 11-28T and without even touching any adjustments, the issue disappeared, gone.

So, not to leave you hanging, I tinkered to see if I could come to a determination of some kind. As it turns out, I was incorrect with the initial tooth count description. It was the 17T to 19T interaction that was faulty. not the 19T to 21T; which is important.

The DA 9100 cassette I have came with cog "17C", whereas cog "17B" is spec'd for the 11-30T cassette in the Shimano dealer manual: going to 19C on the next cog. Hmmm. What adds even more curiosity is that the 12-28T cassette has a 17C-19C combo, indicating that 17C & 19C should work harmoniously. <sigh> Well mine don't. Anyway, it just so happens I have the "17B" cog option from another spare r8000 cassette on hand & I inserted 17B into the 9100 cassette, put it into the wheel & BINGO. No more issues, perfect shifting. Of course, that doesn't explain why my cassette got a 17C, when a 17B is spec'd. My head hurts.....just a little.

I'm glad that bent derailleur hanger was not the cause. The bike is near-pristine & enjoyed perfect rear shifting prior to the derailleur swap, so commentary referring to the hanger was not particularly helpful; but I understand that remote diagnostics based upon an incomplete written description can only get you so far depending upon the issue.

Last note: I converted the 11-30T to 12-30T while I was dinking around since I never use the 11T. Haha, a 1/2 DuraAce, 1/2 Ultegra hybrid: 12-13-14-15-16-17-19-21-24-27-30. Ironically, if the original cassette had just worked, the cool new combo never would have happened.

Thanks for everyone's input.