DigDeal Sorry to hear the firmware downgrade did not work for you. I wish there were better methods for empirical measurements of these problems (I see there are some hopeful developments in other threads here recently). Because of this, it can be hard to determine whether some irreversible changes are occurring during software updates (not limited to the main firmware), or if our brains now associate the whole device with being a threat, even after reverting to the previous, tolerable state. I hope our continued research here will ultimately bring some clarity.
@GBowler noted different levels of eye strain between two identical MacBook models with different installed firmware versions. I also have several MacBooks of the same year (2015), which all had recent firmware versions, but among them I find certain screens more tolerable than others. So, while firmware remains a possible factor, I am currently leaning more toward attributing my personal issues to panel variations.