I'd try clearing the system cache in the recovery menu before a complete reset. If you don't know how to get there, press and hold down the volume up and power button at the same time while the device is powered off. You can then clear the cache.
I contacted Samsung about this, and was told the lower brightness is a protection because your phone is too hot. I would suggest you check your phones temps, and find out what's causing it to work so hard.
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