If you've got a good reason to shave, go for it. Your scalp will be whiter than the rest of your face, but is less apparent in winter since your face is usually much lighter then (I'm assuming you're in the Northern Hemisphere somewhere).
I had no tan line issues, & I shaved my head first in January three years ago. Tan line is not an issue
Should not be an issue. Also if you buzz it down first, that'll give you're head more sun exposure and allow for it to get color. By the time you are ready to shave it all off, it'll blend just fine. I had that same worry, but because I buzzed for a year, the color was there and there was no tan line.
If you are only thinking of shaving because you think your hair is a lost cause I think you are worrying unnecessarily. Your hair looks fine to me. If it didn't I would say.
I think you will always question if you should shave or not. I say give it a shot! It will grow back! But, we all love a bald head on here and once you get used to it you wont look back!
All the best