Floor plan 1.5 stories gable roof without basement - Criticism / Suggestions?

And above all, if the price was contractually fixed beforehand based on a turnkey skill, but the credits due to underperformance were not...

With this post, you immediately open up a big can of worms across many fields about which entire chapters could or should be written. Here I want to focus only on the introduction: fundamentally, the prospective builder must be clear that a general contractor (GU) is the ideal partner precisely when the business basis is: "I don't worry about how many parts make up a whole, and you build me the complete house." If this is pickily varied—"where my buddy offers me a better price, I want to exclude that"—the mutual win-win for the GU loses its shine. And I want to warn very emphatically—especially without detailed planning!—against ignoring my Steinemantra regarding tiles, clinker bricks, heating technology, or the like, and removing trades to cook up special extras against the GU’s standard. A house is—especially when planned as a draft slave—a system in the sense of a sum from which you must not cut! Maybe I should write a book: "Villa Watzlawick—Instructions for a Building Ruin" :) :) :)

This often applies to mailbox general contractors - at least in times of rough waves in purchase prices.

It is not a "mailbox general contractor," so not a GC, but a turnkey contractor.
By the way, one who did not adjust the offer price from November 2020 to now with the acquaintance, even though there was no fixed agreement or the like. Simply a decent company that has its core competence in shell construction and therefore is not upset if other trades are done in self-execution ;)

Is not a "mailbox general contractor", so not a GC,

I did not mean to imply that your GC is a mailbox GC, but only generally to point out that the willingness to carve out lots / trades also depends on how deeply the GC is engaged with providing their own services.

By the term mailbox GC, I specifically refer to those GCs who are often miles away from a "commercially organized business operation." A typical mailbox GC has a ten-hours-a-week secretary (not "secretary," since those who at least can spell the job title correctly are often even skilled workers) and otherwise, as the owner of a rickety mortar bucket transporter, only hires one or two drywall universal finishers depending on the order situation. A GC may very well have no own masons, but instead top-notch, examination-proof, cleanly separated accounts receivable and accounts payable accounting, a first-class sky-blue credit line, and so on. There are even some who have not a single own construction machine, but capital. A GC can therefore certainly run a large operation and act more professionally than a mailbox GC. And otherwise, being a GC or GC is not a question of size, but more a question of principle, whether you prefer to move money or construction machines.

Overall, I read your objections mainly as follows: Better give up a bit more garden space and plan the house bigger(?)

No. You should plan with the neighbors and not against them. They will become part of your life. -> you cannot isolate yourself and you don’t have to. And if so, usually a hedge by the terrace or the garden enclosure is enough. And P.S. Children should grow up, one day stand on their own feet, and live in their own four walls. Or do you want to give the big bathroom to the children and keep the shower toilet for yourselves? You will need the room upstairs later anyway for new hobbies and the grandchildren ;)