Proper piping system & appropriate diameters for sanitary?

Changing circulation loops or opening up new spaces are, for example, significant changes due to the risk of stagnation. Replacing a tap is just like repairing a pipe, not a significant change.

*I meant installing a faucet, as already mentioned above.

Completely redoing the installation in a house certainly qualifies as a significant change.

But this is not about legal issues, rather that this is simply not a DIY job and should not be. And in my opinion, one should by no means encourage the OP to do it.

2 scenarios from my environment:

Case 1) The TW installation is done entirely by a professional company. Unfortunately, a fitting was not properly crimped. The consequence appeared about 1 year after moving in, when water crept up the walls and ran through the seams of the floor-to-ceiling windows.
Consequence: The ground floor was completely gutted and redone. Costs almost €100k, all paid by the installer’s insurance.
In the OP’s case, he is stuck with the costs and faces a financial abyss.

Case 2) The installation was completely done by a craftsman who is also skilled. When he was finished, the water utility refused to install the water meter until the pipes were approved by a registered company. It took him 3 months to find someone willing to do this – with what he then had to pay, he might as well have had it done right away.

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So I can only repeat myself as a well-meaning piece of advice, to have a professional company do it.*

You are of course right. More and more municipalities/municipal utilities require the installation to be carried out by a registered company. Few people feel like tearing out screed or having rainwater in the drinking water. If you are not familiar with an industry, you should get help.

If a plumber comes to you tomorrow and says he wants to teach a 10th grade class in German and history starting next week and needs quickly the material that is currently taught and how to convey it - what would you say to him?

"Sorry, but I am a social educator and not a teacher."

Case 2) The installation was completely done by a craftsman who is also skilled in handiwork. When he finished, the waterworks refused to approve the installation of the water meter until the pipes were inspected by a registered company. It took him 3 months to find someone willing to do it – with what he then had to pay for that, he could have had it done right away.

Well, that sounds like a viable alternative – only that it is best to arrange a suitable company in advance to discuss the conditions and requirements and after the installation has been completed, have it inspected and approved.