I'm curious how bright most people have their iMac screen. I also have a late 2015 27" iMac, but since I mainly work on photos for print, my standard iMac brightness setting is five blocks up. I also use a hood to block any extraneous light from falling on the screen.
There is similar tread for this screen type, just with pink edged. For me apple authorized repair center quoted 700€ screen replacement.
For retention i would maybe start using screensaver for 1h or so to “reset” pixels...
This happened with mine even before Apple Care ran out on my late 2015 27 5k. It always went away as @dcmontgomery said. Screen brightness is always maxed.
There's no fix to the pink edge or the slowness after the Catalina upgrade.