I bought a few shelves and some wood at the hardware store. Then I cut them to size and built them into the niches, filling all the gaps and later labeling and placing boxes there.
Short rails with shelf brackets, e.g. from the company Dolle. Attach the rails using a wooden wedge underneath, deep brackets and shelves at the bottom and shorter brackets towards the top. This way you would have used every centimeter.
You often find something like this on the internet:
It’s not off the shelf, but putting something like this together basically can’t be much more effort than the Ikea shelf. :)
Otherwise, I also think the half-height heavy-duty shelves are a good idea in terms of price-performance.