Do we really need this thread? I have a suggestion for the OP. Use your iMac as you would normally use it? How do you find the display when normally using your iMac?
Quoted for truth - with the modification that this looks normal for an edge-lit IPS-screen. If you use mini-LED backlighting, you avoid this.
Note that with mini-LED you will still be able to film/photograph glow with a good camera, but the ability of the eye to discern light levels depends on angle of view. At smaller angles the high intensity light blinds the eye to nearby low-light contrast. Thus micro-LED or OLED will look better when filmed with a good camera, but it may not matter to the eye, depending on how densely the backlight LEDs are distributed. YMMV.
As has already been said, having a completely black screen with the backlight brightness pulled up really isn’t a normal use case. Or even a use case at all. The phenomenon is mostly visible when watching dark scenes in movies in a darkened room.