Table, chairs, sofa... where on earth to buy?

For example, I consider spending more than 150 euros on a dining chair absolutely excessive

Yes, that was my limit. However, based on the reviews and my experience, you can also tell from the padding and the fabric whether something is of higher quality.
You can especially recognize quality by the fabric. Just as a 1000€ Neckermann kitchen has a melamine coating, a brand-name company has lacquered wooden fronts.
... and without children, you can also place a bit more value on quality or design. I'm a bit of a penny pincher, but it can still be a stylish individual piece for me – I place a lot of value on aesthetics and good living. It simply has to fit. If there's something really great, then I'm roughly willing to spend 20% more. After all, I look at it every day. But everything has to match the salary.

...If there is something really great, then I am willing to spend about 20% more roughly. After all, I look at it every day. But everything has to fit the salary.

So something is definitely going wrong there ;) You often give (in my opinion) qualified answers to the questions here, know how to express yourself appropriately. That qualifies you for more than a job in the low-wage sector. Ever asked for a raise? :p Then a Thonet will also fit sometimes.

That qualifies you for more than a job in the low-wage sector. Ever asked for a raise? :p Then sometimes! there’s also a Thonet chair in it.

Joker :DTariff tables in the authority unfortunately don’t offer much. I’ve been in my final pay grade for 20 years and go on strike every two years for a few percent :p My task, among other things, is to write reports so that everyone can understand them ;)

I have already ended up with the Dutch providers...

We found what we were looking for on a trek through Holland at refri in Weert. Unfortunately, they are currently hard at work on a new version of the website and there is nothing to see. Here one of the chairs is placed at one of our desks. Very comfortable and stable despite thin legs and casters. The manufacturer is unknown to me. There are many upholstery options – we chose a non-animal covering reminiscent of suede or Alcantara and in addition to red we also took blue, green, mustard, and gray. We found €275 fair for the quality. Since the chairs are made to order, you need 6 weeks of patience.

Thank you. A similar (pink) one stands on my desk :p
99€ at Westwing. With yours, I would be afraid of cutting off the circulation to my legs if I bend them over the armrest :)
But good to know that the spindly legs are more stable than they look.

Your suggestions are good. If you enter the stores/companies/designers in Google, more similar shops are suggested that are worth a visit.

I’m a bit annoyed: every year I face the same challenge of finding a special piece for the house and replacing old furniture. Without Corona, my husband and I visit the usual furniture stores to leave the uniform mush behind us. Then come the Hamburg specialty stores: small, fine shops. One more expensive, one cheaper. Meanwhile, we Google. One insolvent, another moved or newly opened.
I hate lists on the computer, I don’t like favorites. You don’t even look at them anymore anyway.
I’ve now created a permanent list of “my” stores by hand and will visit all the stores again tomorrow on my day off, considering my increased budget.
I’d probably have an easier life if I liked the uniform furniture store mush where everything matches :D

But good to know that the spindly legs are more stable than they look.

They have to be, because my weight is significantly above average. The greatest strain actually comes from rocking on the chairs, and with the casters underneath, that is a somewhat inadvisable art.

I would probably have an easier life if I had the furniture store generic taste where everything matches.

That's what makes the hunt especially enjoyable and the hunting success rewarding. Just take some time – if necessary, simply occupy the chairs longer until mobility returns.