@Catalyzt: the sign of a bent derailleur hanger was I could not reliably shift in the middle gears. Who knows what's happened to yours.
VERY interesting. Would be weird in a bike that just came out of the box... read on, I don't think that's what's happening, I think I got this sorted, BUT I will definitely keep that under consideration if the problem repeats under greater load.
I dug around for a bit, and everything that I could find tells me that your derailleur is mechanical, not electronic. Does it have a small diameter cable (wire) that can be unplugged from the derailleur? Di2 does. If there is a twist knob at the end of the cable that fits into the derailleur, it is mechanical. This is a worldwide forum, so who knows what Shimano sells that may not be available in the US, but I have never seen any Di2 rear derailleur that has a mechanical tension adjustment (not including the B-screw).
Okay, appreciate that-- I feel a little stupid, but not too stupid; with that freaky shift-lever-inside-the-right-brake-handle system, I could not see any cables except the ones exiting the frame right by the derailleur, and I read-- somewhere-- that shifting was electronic, so I assumed it was.
And I kept thinking, "Wow! This electronic shifting sure duplicates the feeling of mechanical shifting so precisely! Is that a bug or a feature? And if it's a feature, what is the point of electronic shifting?"
Duh! No, there is not some super secret system of servo motors inside the bike using AI to duplicate the resistance and tension of manual shifting. Of COURSE it is manual. Thank you for being kind when you pointing this out!
Update:
--> Clutch is good, or good enough, passed finger test.
--> It appears the cable was too slack.
--> Four clicks anti-clockwise appears to have solved the problem, at least in my cul-de-sac test.
(I am now having to explain to the neighbors that I am not insane and my intention is not to ride the bike in circles endlessly. BTW, if you ever do this, look up periodically and focus on your surroundings to avoid dizziness and vertigo.)
Friends don't let friends ride bikes on acid.
I don't need acid, apparently, I get high / dizzy just riding my bike in circles like a trained monkey. It's like early sobriety, when I would mimic, briefly, the feeling of being high by wearing glasses that were the wrong prescription.
Shifting appears nearly flawless now-- might be a tiny bit of residual chatter in sixth or seventh, but it's so subtle I could be imagining it. Will take a run this afternoon and see how it does under slightly greater load. (I avoid shifting under extreme load anyway.)
Thanks, everyone! Super helpful!
--MJJ