I dislike built in shelves or niches in showers they are almost always the source of leaks , and IMO shampoo bottles are not decor items . Not to mention those shelves usually get gross You can get anice looking shower caddies that go over the wall or door. and are easy to clean. We just take our bath stuff with us when we shower and put it back in the cabinet when done . We aslo have international students that share a bathroom they each get a caddie for all their needs they take it back to their rooms when done keeps the clutter to a min. I do have a cady over the wall in our bathroom that holds what we bring. As for yiur shelp placement think about how you will reach the items in the spray from the shower or the opposite wall

If you're set on the shelves, they should go on opposite wall of shower valve.

Put them on the side that won't get sprayed with the shower. 12" separation is not enough to keep air circulating around the bottles to prevent mildew. Have you considered one in each corner? Shoulder height is good. Get your shampoo bottles out and try out some arrangements.

I would do a recess, not shelf, but always on the valve side.

Keep all the mess and distraction on one side.

I have glass shelves in both my bathrooms. No problems at all. Hate niches just scummy messes IMO. They are opposite the shower head etc

I agree that horizontal shelves are better than corner shelves as far as space.

Look at clear plexiglass options. They won’t interfere as much with your beautiful design decisions.