Additional positions in house construction: are they useful and reasonably priced?

I read that quite a lot of fittings, etc. are to be provided as a service by you. Especially for showers, etc., you should have in mind that the type of fitting must be coordinated early on, e.g., concealed, so that it can also be installed by the general contractor in the appropriate place.

And, regarding the 50 euros on site for the drywall installer and the niche in the shower ... this covers the niche in the drywall, but it is not yet tiled. Tiling the niche will account for the main part of the additional cost charged by the general contractor, as this means increased effort for the tiler.

Is the drilling always deep enough for the deeper boxes anyway? So is it only about the plastic boxes themselves with such a "upgrade"?

No. The electrician drills as deep as necessary.
I doubt anyone nowadays still struggles with the standard boxes; the deep boxes are simply more convenient for the current type of electrical installation. That saves time during wiring, time is money, etc.

Of course, I can't rule out that your general contractor has a penny-pincher on hand who actually drills and installs only 46mm deep. The effort to replace that would obviously be huge... chipping out old boxes, somehow pushing the cables aside, drilling deeper, plastering in new boxes...

I only said that I find the surcharge cheeky. If you have to pay it to guarantee 68 deep boxes, then unfortunately that’s how it is.

(...)
And, 50 euros on the construction site for the drywall installer and the niche in the shower ... this means the niche is made in drywall, but it is not yet tiled. And tiling the niche will take up the main part of the additional cost from the general contractor, because that means increased effort for the tiler.

You can also talk directly to the tiler.
As soon as a general contractor is involved, the price rises exorbitantly, because of course they have effort with it and also want to make a profit.

Not good advice. I'm not even annoyed by the tram, which currently thunders past my window about 50m away.
At least take the decentralized ventilation. If it annoys you too much, you can always pull it out later, insulate, plaster, then it's quiet.

Installing it afterwards is much more effort and, above all, dirt.

You are right. Better some ventilation than none at all. My advice would be more complete: central ventilation (sink 4K to shut it down, but I wouldn't find that irritating either ;-)
I have now heard multiple times about decentralized ventilations praised as whisper-quiet. No way would I install those in a new house ("noise" makes you sick).

You can also speak directly with the tiler.

Well, tiling seems to be the responsibility of the general contractor otherwise, and if you want to realize a niche like that, you have to communicate properly with two trades at the right time. I'm only thinking of the thread we had recently: ... something should also happen on the builder's side. And here it’s about a total of 340 euros compared to an as yet unknown amount for the two executing parties in solo coordination.

Sure. But he wanted an assessment of the costs. And I find 340 EUR for a 30x60cm niche in the shower outrageous, nothing more, nothing less. Even if that would mean 1m² more tiles due to waste, etc., plus half an hour more work for the tiler, plus half an hour for the drywall installer... we would end up at 150, at most 200 EUR. The rest is pure profit for the general contractor...

Because if you pay the tiler 100 EUR on the invoice, you can be sure that your tiler gets at most 80 EUR when he does the same work for the general contractor.

PS:
Net prices, of course.