Construction costs are currently skyrocketing

So: Oak will become scarce and expensive once the often still available stocks are depleted.

We ordered solid Swiss pine for the kids' room decadently last week. :D

So, quickly select the sample for the stairs ^^

That’s probably far too late... if you select the sample for the stairs today, they will be installed in 7 or 8 months at the earliest, if at all...
The stair builder only purchases the material shortly before production; it sits with him for months... the sampled stairs won’t become more expensive if you have a contract with a fixed price. But if he can’t get oak, he can’t deliver.

With this opinion I will probably cause some friction, but how he then installs it is none of my business or my problem but the problem of the main contractor (GU)... That’s why I work with the main contractor so I don’t have to deal with stuff like that...

Although this opinion probably annoys some, I really don't care how he installs it—that's the general contractor's problem, not mine... That's why I remodel with a general contractor so I don't have to deal with stuff like that....

We started with this approach as well. But it didn't save us from frustration. In the end, I no longer wanted to accept that the general contractor turns his problems into ours. Now I do it myself after all.

This place is teeming with experts. Are there any real ones too? :D

Building materials are scarce because they are building like crazy in Asia...

They are no longer building in Asia because the real estate crisis there is even worse than ours.

now you may again… ;)

Although this opinion will probably cause some friction, I really don't care how he installs it, or rather that's the general contractor's problem and not mine... That's why I work with a general contractor, so I don't have to deal with such things....

This is the completely correct point of view, I see it the same way. Only, if he doesn't have the material, he can't install the staircase, no matter the price.