My current hypothesis on the flicker: I have an M4 iMac and have turned the auto brightness off, and get the flicker seemingly when certain images are displayed on the screen (brightness increases then seems to decrease again when whatever image has triggered it is off-screen). When this happens changing screen brightness manually gives a similar flicker rather than the normal smooth change. A reboot will stop the manual brightness flicker for a while, and whatever has changed as a result probably makes the image-related brightness changes unnoticeable as I don't see it for a couple of days.