the detail is common and widely accepted... I call it A "proud reveal"...

I have not seen this technique before, I wonder if you can have only the bottom piece of Caesarstone protrude a bit so that it looks more like a shelf. Just a thought!

Here is an example, notice how the jambs and sil are "Proud" of the wall tile exposing the finish thru body edge of marmara ? A proud reveal, we use this in Niche's too...every situation and design is unique.

You don't need much depth for the shampoos..Here are some photos that you might find helpful

Thanks A Kitchen Design. I see that printesa posted a picture of a niche where the shelves do stick out a bit, like you suggested, and the rest is flush. Also, looks ok. Since those surfaces will be sloped a bit, should be no issue with water puddling there. Printesa, you also posted a picture of a niche in a dark tile with white jambs. Can't tell if that one is flush or not but that is exactly what the one in our tub area will look like (dark tile, white Caesarstone jambs). Because that one will have a tile back and will be more decorative than anything (no large shampoo bottles!) it should turn out the same. I will definitely measure all of our "stuff" to make sure it will all work. Thanks to both you!

We are doing a niche for one shower and we'll use porcelain tile. The grout line can be very narrow if the tile is cut at 45 degrees. I see these
Here are some more ideas The last one has the protruding niche shelf