Hi B C,
I can't speak authoritatively for the SE 2020, but I can tell you for sure that a bad camera on an iPhone 7 Plus can keep it from booting. I had to replace the camera assembly to get the phone to work again; same symptoms, it worked when the camera was unplugged and putting in a new camera fixed it.
I don't think the camera is required for True Tone, but the ambient light sensor probably is. Since they're on the same connector there's no way to tell which would be the cause of the problem.
Definitely time for a new camera. Fortunately it won't break anything on the SE 2020 to replace it, at least as far as I know.