I like a simple plan for bathrooms a 12 x24 porcelain tile for bathroom floor and shower walls with a smaller tile for the shower floor but in the same color family. Then the counter can be the star.

Oh I love that turquoise tile!!! Make that the star.

the turquoise will be in our niche -- too expensive to make it more than an accent ;)

any suggestions on how to best incorporate our countertop?

Your designer gave you good advice on the solid pieces. I would change the blue glass to something smaller for a niche; the color is fine. The 2x8 is more contemporary than the gray tiles.

My main concern for bathroom and shower floors is always safety: non-glossy tiles in a size which allows your feet to always be in contact with sanded grout. A bathroom is full of hard surfaces so slipping and falling is a real danger. I also want a shower/tub with a non-slip bottom or an added rubber mat for safety. When traveling, if the shower or tub/shower floor seems slippery, I put a bath towel down on the shower floor for added safety.

The turquoise tile is pretty with the others. You could use it as an inspiration for wall paint. If you think it is too much for the whole room just do one wall. The one behind the vanity/mirrors could work nicely

Is the subway tile the shower wall? I’m assuming it is in the white family. Can you post a photo holding it vertically so we see how the light hits it. You can install it lots,of different ways to get more interest in the shower walls.
And yes, glass tile that size is probably not great for the niche. What about turquoise penny tile for the niche? Do you have trim tile for the niche to match the wall tile? Perhaps Beth can chime in with ideas.

Yes, the subway tile is for shower walls. It is a light gray -- when light hits it right, it appears almost white. Here is a larger sample of it with some more light on it: https://waynetile.com/wp-content/uploads/2019/01/SliderGRIGIO-ASH.jpg

The niche tile is the color I showed, but a different pattern. I think it will be small enough. Here is another houzzer I used for inspiration: https://www.houzz.com/hznb/photos/alhambra-valley-residence-contemporary-san-francisco-phvw-vp~941904

I like the idea of a white quartz around the shower sill..might be tough to match the marble tile color/veining with a countertop -- better to not try? I guess I'm nervous about having 3 different tiles, and 2 countertop colors/styles/textures in the room. Don't want it to look too busy

You an buy a marble saddle for any doorway, so that can also be used for the shower entrance. I think it will look just right with your floor choice. Diana makes a good point about the niche tile being blocked by bottles of shampoo....