At the risk of sounding like Steve Jobs, there's one more thing: The native contrast in Big Sur was too little for my eyes, so I increased it in System Preferences > Accessibility > Display. That solved the issue.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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