No to the gray , please? A nice white tile that matches the white of your quartz. is more than enough.

Go shopping locally, drag home the sample boards, stand them up. Is there a better way to select? No, there is not

You have a multi white/gray tile back splash now, you want to expand it to the black band below the raised counter? More tile isn't availible?

Won't comment on the backsplash right now but will say that your kitchen cabinets are so dark that any detail they have is lost (at least in the photos). Would you consider painting them a lighter color?

I would consider painting cabinets, color suggestions?