Get an S9, unfortunately.
That seems to be the only answer I can find.
The S9 has a toggle to turn on/off auto brightness, under settings, lock screen, Always On Display.
The toggle isn't there on our S8 and S8 plus phones. :(
It's simply missing ... such that there is no place to turn off "auto brightness" on our S8 generation.
It seems we are stuck with having to live with an incredibly dim AOD such that one can barely even see it, unless you light a Bic or shine a flashlight into the sensor EVERY time you turn off the screen.
That tricks the auto brightness into making the AOD as bright as possible, such that it can ACTUALLY BE SEEN! :-P
What's the sense in that.
Sux too man, because ive downloaded all kinds of perty kewl GIFS to set as a moving AOD image.
My fav is the Pac Man screen... but one can barely see it. (Unless I shine a light into the sensor just before AOD activates.)
Sorry to be the bearer of bad news.
I VERY MUCH hope my answer is incorrect, but I've researched the heck outta this, been everywhere in my S8+ settings, and I'm 99.9% certain us S8 owners don't have the option for anything but a default "auto brightness" with no way to disable it.
PLEASE GIVE US THE ABILITY TO TOGGLE IT OFF, like on the S9!
Have a good day everyone,
And be groovy.
I think you're right. This option was there on android 7 and got removed with android 8 and never came back.