Ok, so if it doesn't look like the pictures and you can adjust the picture setting to your liking when you're actually watching something, personally the brightness on the TV's menu wouldn't really bother me. I don't spend that much time in there anyway.
Each person has their own tolerance for annoyances but compared to your brightness issue, I personally have so many more problems and annoyances with my own system including issues with the TV, a soundbar, my cable box, my Apple TV 4K, my cable company's app, etc... that if a bright main menu was the only thing that annoyed me, I'd consider myself lucky.
It's hard to tell from the pictures but just based on those pics it looks like the TV has a really bright hot spot in the middle of the screen that shouldn't be there. Mine definitely doesn't look like that.
I think it is the Graphene wallpaper specifically.