Buy tiles cheaper somewhere else?

Yes, I pay the construction company. In the end, though, it doesn't matter. The company certainly gets the tiles at a completely different price. The question is whether they pass on a certain discount, at least partially.

No, it is not irrelevant. How should the dealer know at what price the construction company sells you the tiles? If you build with individual awards, you might also go to a showroom of a wholesaler or intermediary dealer. They show you tiles and can tell you what the normal selling price, so to speak the recommended retail price, is. But whether your tiler gives you a discount and if so, how much, the dealer does not know either. It shouldn't be any different for you. I still miss the answer to the question whether you have even spoken with the construction company about the surcharge... so it sounds like not yet. If not, the discussion is pointless. Ask your construction company what surcharge they charge for the tile, everything else is nonsense. You are speculating here about a hypothetical surcharge.

Here functionality (wet room = moisture) meets aesthetics.

No, here moisture meets "this is how it was done in the past". For functionality, it is actually better not to tile everything up to the ceiling. Tiles cannot absorb moisture. That is why most tiles are only installed up to the ceiling in the shower or to a certain extent. Everything else only as high as necessary (splash water/cleaning).

That would not have been the case with my GÜ, by the way. He would have actually passed on all the tiles and/or sanitary surcharges 1:1. Or if up to 20 EUR were included in the construction service description (which was not the case for me), he would have just deducted that from the dealer’s MSRP. Whether he actually paid the price or not, or whether I paid the dealer and the GÜ received a commission, I don’t know and didn’t care. In any case, everything was way too expensive, so that’s why I’m doing it now by self-award/self-performance.

yes, but now that is the exciting question. There are GCs who make a golden nose with the inspections and get their extra margin here, others do not. But as long as you don't ask, it is speculation.

Yes, that's true, especially since this was handled differently for each trade even with my general contractor. Civil engineers, masons, windows, and sanitation were handled through the general contractor. The staircase and doors went through the staircase and door builders respectively. The door builder even wanted the extra charge for the customizations in advance, which made it quite easy for me to kick out that trade. is right, ask your house builder, but have a tissue ready.

By the way, that would not have been the case with my [GU]. He would have actually passed on all tile and/or sanitary surcharges 1:1. Or if up to 20 EUR were included in the construction service description (which was not the case for me), he would have simply deducted them from the dealer's RRP.

[GU] solid house/ prefabricated house manufacturers often differ very much according to my many years of forum experience as follows:
[GU] gives the craftsman in interior finishing (electrician, sanitary, hairdresser, drywall installer, and stair builder) free rein when it comes to upgrades.
The prefabricated house builder gladly handles everything through their accounts.

The same goes for the tiles with the [GU]: everything that is discarded goes through the tiler trade.

Live and let live. Yes, the company is top, also affordable, just as you would expect. This is confirmed by everyone we know and even those we don't.

Then just look at the whole thing and don't get tangled up in details. If the package is right, then it's right, you don't have to nibble away at the margin just because you think you clearly recognize it in one position. If you want to negotiate positions individually, you shouldn't buy a complete package.