ᐅ What type of shelf or storage is recommended in a shower for a new build?

On the page from my link (also just googled) these units are also available for retrofitting.

Pwnage619 schrieb:

The shower niche wall needs to be made thicker, which would reduce the shower size to about 75-80 x 90 cm (30-32 x 35 inches), instead of the original 90 x 90 cm (35 x 35 inches), and we don’t want that.

I have already read your builder’s argument, but I have my doubts.
A drywall partition with a stud frame should be at least 15 cm (6 inches) thick. Cutting a hole somewhere, boxing it out, and then patching it up to create a niche 10-12 cm (4-5 inches) deep shouldn’t be a problem, right? Or how thick is your wall? Or is there just one wall and it borders an adjacent room? In that case, I would understand.
Have you uploaded your floor plan anywhere?

As Tolentino writes: If you take the wall where your shower rod is mounted, the water pipes are probably located behind a drywall partition rather than recessed directly into the masonry? In that case, the drywall typically extends all the way to the ceiling — and where the shower rod and hose connections are not in the way, you can have the drywall installer build a box that the tiler can then cover with tiles.

Here is a picture showing how it looks on our side – the drywall partition would have been built either way:

There are also ready-made niches available in stainless steel, some already tiled, or options to tile them yourself.
These tend to be more expensive.
I also agree with Tolentino that there’s really no reason against these niches, and the walls don’t need to be made thicker unless you have unusually thin drywall.

We will have niches in the shower and above the bathtub. Additionally, there will be some as storage space next to a bed (which is against the wall) and above the sofa. I’m looking forward to it.

Consider placing the riser pipe inside the wall, which keeps the entire wall surface free in front of you. Depending on the shower, for example, a narrow glass panel could be installed continuously along the wall. There are also many options for chrome shelves from various manufacturers like Keuco and Giese. It also depends on whether you prefer the classic solution, with one shampoo bottle for everything, or multiple small tubes and bottles scattered around. After that, I would choose the shelf accordingly. The recessed niche solution definitely works just as well, so why not go for it?