I've been struggling with this severely throughout Dawntrail's MSQ too. Everything is just very flashy and there's a lot of eyestrain and photosensitivity related issues this time around. I deal with migraines and seizures, and I've had to permanently leave the Battle Effect settings on "Show Limited" because of this. Viper and Pictomancer are way too intense. Several of the areas are just way too flashy and full of eye straining effects, especially the Dawntrail zones.
Many of the dungeons, especially if they have moving effects or strong bloom or visual effects, cause so much eye strain that I've had to look away or just... stop playing. I've had to run a few Dawntrail dungeons with Duty Support so that I could go at my own pace too because I have no idea when a boss battle is going to leave me unable to continue.
Has anyone had issues with the final trial to Dawntrail specifically, however? It's so bad it delayed me finishing the MSQ for a week. I couldn't complete it the first time because it left me with a severe migraine, and had to abandon the group I was in because I was afraid of it getting worse. I was eventually able to complete it thanks to some suggestions to minimalize the visual impact (playing in a window with all the brightness settings at the lowest, and using the Windows Night Light feature), and the aid of a full group giving me a danger dorito so I could avoid looking at certain screen effects. But it still left me with a pretty bad headache.
If there isn't a way to reduce the battle effects and deal with the eyestrain, I'm probably going to have to significantly reduce how much I play FFXIV or stop altogether. Which sucks, but I don't really know what else to do at this point. :\