@DavidAnderson My mac is now upgraded, including a further minor update to the latest version of Big Sur after the move from Yosemite, and works perfectly. How can this possibly be anything other than successful upgrade to Big Sur? If it "failed", would my OS not still be Yosemite (which it is not)?

@stephanmg See my answer

@Arefe my problem was with the laptop’s screen. I don’t actually use a second screen.

@SteveChambers thanks for the suggestions. I tried each one separately to scientifically determine what fixed it. Unfortunately none of your suggestions made any difference. Disabling True Tone did the trick (see answer).

Option+Command+Shift+V doesn't work with Excel v 16.49 on Big Sur, but it does bring up the special paste type selection dialog, so only 2 more clicks to the finish line.

I haven’t done it yet. Will definitely post if success or not.