Shoes and jackets on 1.37 sqm - realistic?

That calls for a DIY project from you :). Or expensive custom-made, otherwise it will be difficult.
We also have a tricky area in the bathroom. A niche 270x95x35cm.
We hid the heating circuit distributor there and on the left ventilation pipes behind cladding.
I ordered the cabinet made to measure and put the rest together myself :D. Didn’t want to waste any space.

That looks really great. And the wallpaper to start with. Yes, this will be a DIY project for us. Now I've just realized that we first have to prime and paint...:(

Thank you :) . Either you order shelves/cabinets that roughly fit and then put something together yourself, or if you don't have suitable machines, you could possibly order custom cuts to measure. There are plenty of online shops nowadays offering all kinds of wood types with different decors. Then you would only need a cordless screwdriver, drill, hammer, tape measure, try square, and Forstner bit for the furniture screws to connect the parts. Overall, that's almost all there is. You just have to keep in mind that it's best to secure your shoe rack to the wall with brackets, otherwise it will be wobbly.

... by the way, finished for almost 3 weeks:
The wardrobe for 2! people under the stairs!

Primer, pine wood (before it became scarce), simple shelf rails, clothes rail and glaze for the boards from a total of 3 hardware stores (Corona time), high-quality leftover paint, Faktum cabinet and IKEA cubes from the stock. 2 days of work and now lots of joy. Right wall still has potential... IKEA currently charges 39€ for delivery, so some things still have to wait.
Summary: from about 28 spaces for pairs of shoes now... I can hardly count them :D, definitely enough and space for empty bags in the corner... see for yourself: (everything is straight, the crookedness comes from the pano shot:)

What should one advise the advisor...

I have this hybrid idea in mind. The carcass and exterior could be made by a carpenter in a joint project discussion; he has the tools to make sure everything fits and holds – and the boxes aren’t the expensive part in furniture making. You create the internal divisions yourself and fit them according to the drawing. It’s not rocket science – even with common tools. This way you could achieve the following:

[*]Full utilization of your creativity without grid limitations
[*]Remaining within an acceptable price range
[*]Durable and good solution for your family

Second thought: We have a MYCS shelf, which is really good quality. We had doubts but were convinced beforehand in the store. The 30 cm grid is possible, the assembly uncomplicated, the color design variable. All good. Only the drawers are very special space wonders – astonishing how little storage space can be behind a large front! The configurator online is great.

What should one advise the advisor...

Not that we misunderstand each other: A) it’s too late for advice now and B) I find a carpenter/joiner a bit exaggerated for storage rooms.

We have a MYCS shelf, which is really good quality. Had doubts and convinced ourselves in the store beforehand. The 30cm grid is possible,

With a 30cm grid you massively limit the possibilities. For example, if I were to resort to a 30cm grid, I would get 4 pairs of shoes side by side. Or (again) cluttered compartments.
I googled these things once after asked her bathroom question and the bungalow atrium builder posted his living room corner. They look nice, but I wouldn’t see a need for them now in a closed storage room. It would be like committing a crime scene in high heels and a ruffled blouse :D (see above!)
I have such shelves in no-name and Ikea and wouldn’t need to buy new ones now. I prefer to go for individual pieces ;)