I don’t want the whole screen to get darker when the interaction icon appears - If I could tell it not be a part of the post process auto-exposure calculation, I would avoid this issue.
There’s no simple way. Probably by using some custom Post Process material with Custom Depth masking.
Of just by using 2d widget instead of 3d effect (those should be still not affected by post process).
Also the brightness of the material bilboard is very low (the color is multiplied by 0.1) . The whole problem with the eye adaptation on the mat bilboard or the 3d widget is that in lit areas in the level, the mat bilboard looks dark, but in places where there is no light it looks bright, so the eye adaptation has to compensate.
Actually the way to show 2D widget in space (to be able to put it in the scene) is to use 3d widget (the 3d widget has a slot where you can choose which 2d widget to use). I am not aware that there is any other way to place (to make the 2d widget to “float”) a 2d widget in a level.