The Lugarno looks too big because it is too big. I went down that same path and ended up returning several RH and PB bath scornes. The problem, I finally concluded, is that they are designed for bathrooms, not powder rooms, and large scale bathrooms at that. PB has one, the Nash Candlestick, that might work for you. But do take some time to look at other sites or in your local lighting shop for sconces that aren't made for bathrooms but for dining rooms or halls. The scale will be smaller and there will be more designs that "speak the same language" as your house and your design plan. (Which BTW looks great. I think the gold mirror would look terrific with polished nickle fixtures and keep it from being too matchy, matchy.)

I think the gray walls with a white ceiling and trim will look fine. You can reuse the mirror and have other black elements in the room ( knobs on new vanity & flooring). When we renovated our bathrooms I chose Sherwin Williams Mega Greige and it looked really nice with white sub-tiles and white wainscoting. It made the ceiling appear higher. We left the ceiling white. The tile flooring had some whites, grays and browns marbled through.

It looks like your current paint color is pretty close to the tile color, which makes the tile less noticeable. You could try adding more color with some colorful towels and decor. Or you could still paint the wall to add more color but it might end up making the tile more noticeable.

As mentioned above the tile floor is gonna be the "issue" only because it would really limit the colors on the wall or even the vanity.. are you looking to only paint or is this gonna be a full remodel?