Sadly we are not able to help via addons here
Addons cannot affect the visual effects of the game, just change or add some textures, controls.
And this is also restricted: We cannot change the 3d world inside dungeons, only outside (overland).
So all one could do via an addon would maybe "overlay the whole screen" with some kind of black/grey rectangle as a kind of a filter (if this would help somehow).
And even then it'd be hard to find the correct "trigger" because not all effects are "announced" by the game somehow...
I'd talk to ZOs by the official forums or maybe ZOs_DanBatson here in the forum would be able to forward this request so you guys would get somethign to ease this and make the game enjoyable for you again (like official settings to lower effects without having to set the graphics to minimum).
Currently there will only be that option I'm afraid: Lowering graphic settings so that some effects won't appear anymore (if this is possible).