You should get more help if you post in the Home Decorating area. I am not very familiar with SW colors. For the ceiling you could compare their ceiling white with your trim color and choose between those. If you like Steely Gray but worried that it might be too dark, you could get it mixed at 50% or 75%. I would get samples and paint sample boards to try out the colors in the room. Also consider what color towels. You have a lot of dark but also a good amount of white in the room.

If your grid is accurate and you are 1 sq = 1 ft, then you don't have enough room between the newly expanded shower and the wall for the enclosed w/c. Your diagram shows 1 ft. Also the pocket door won't fit as shown. Consider a wall between the toilet and the vanity without the rest of the enclosure if you are seeking privacy there. See how this wall between the vanity and the toilet serves to hide the toilet a little bit on these two pics below? This is a different layout, but similar concept for the dividing wall. In your space consider only having the wall right next to the vanity and not going over to the other side of the room. A wall to the ceiling would conceal the toilet area and not close in the room IMO.

Pretty! I am doing a similar tile in my bathroom remodel right now. Are you only open to Sherwin Williams? Because Benjamin Moore has some beautiful gray bathroom colors, better than SW in my opinion. I’ve done a lot of research and made a lot of mistakes lol. Here is a good article. I’m doing gray owl, myself. https://www.kylieminteriors.ca/the-9-best-benjamin-moore-paint-colors-grays-including-undertones/

My DS remodeled a hall bath with very similar colors as you are showing. It was an upstairs shared hall bath, shared by 2 girls and occasionally used by a guest. Her shower tile was called "denim," a nice blue like yours, the countertop was marble and a marble hexagon floor, with white vanity, so overall the same color scheme as yours. She actually used a very light, muted coral color that I know was from SW but I don't remember the color name. It almost looks white in the room with just a whisper of color; just enough to see the difference between the paint and the white trim. It doesn't read too feminine bc it's not not pink and it's not bold or strong. Orange and blue are on opposite sides of the color wheel and look really nice together if you choose the right shades. That doesn't mean you need a "screaming" blue and orange combo! Her combo looks very nice bc nothing is very strong. It looks soft, pretty and inviting. You could probably accomplish this "barely white" with other colors as long as they are light and muted. If a color doesn't appeal to you and you want more of a white, test out SW Crushed Ice and other cool whites. Your lighting will make all the difference to how the paint actually looks in your bathroom so try out some samples and look at them during different times of the day.