Lay electrical wiring yourself?

The first question is can you do that and do you dare to work with electricity? The second question is how long do you need for it compared to the electrician and what happens if you don't meet the timeframe and other trades then have to wait? Also, I would not use 3 x but 5 x cable.

a+b) you can't say that so generally. A very common and often made mistake in electrical installation. The length of the cable also has an influence. Roughly speaking, if you go beyond 18m in length, you either have to use 2.5 sqmm or protect with 13A and then you can't calculate 12 sockets anymore.

c) here too, pay attention to length and cross-section.

f) you could philosophize about that for weeks.

The installation guidelines must be observed. Distances between power<>data, current carrying capacity, lengths and cable types, etc.

For example, I had every double socket in the kitchen individually protected. I would first look for an electrician who agrees to do it. Then discuss exactly what you will do yourself and what he will do. Then ask what you can save by doing so. Then take it from there.

The first question is can you do it and do you dare to work with electricity? The second question is how long do you need for it compared to an electrician and what happens if you don't meet the timeframe and other trades have to wait? Also, I would not take 3 x but 5 x cable

Good morning, I definitely dare, but someone wants and must be there to advise. There is a reason why this profession takes 3 years to learn. And the second point I am still pondering is the fear that I might misjudge the time required (as too little). Especially with electricity, of course, as you say, everything comes to a halt then.

a+b) you can't say that so bluntly. A very common and gladly made mistake in electrical installation. The length of the cable also has an influence. Roughly, if you exceed 18m length you have to either go with 2.5mm² or fuse with 13A and then you can't calculate 12 sockets anymore.

Yes, that's true, I always had an influence of cable length in mind from 20m. It should also initially serve as a rough guideline, except maybe for living room and kitchen I don't see the need for that many sockets. But you are right, that has to be looked at and calculated carefully and I am realistic enough that my knowledge as a trained car mechanic certainly is not sufficient.

It seems that nobody from the forum has experience with the internet company either, so that one is out anyway.

We also once inquired about what could be saved by doing work ourselves, such as preparing sockets. The savings then did not seem proportional to the effort for me. We then had everything done and have been very satisfied with that. The technicians on site worked well and gave good advice on installation and positioning. When measuring through, it turned out that another trade had probably damaged a cable; a good and quick solution was found here as well. If we had done the work ourselves, we would have been responsible for fixing it. But of course, that also depends on the quality of the craftsmen.

It seems that no one here in the forum has experience with the internet company either, so it's out anyway.

Why necessarily the internet company? The materials you need are limited as long as you don’t want anything extravagant. For simple switches/buttons/possibly motion detectors, sockets, and LAN cables you don’t need anything extraordinary and online you will find the cheapest prices.
The most important thing is an appropriate company that is ready and supports your project.
If you have technical deficits, then you will get help here.
At the moment... I have learned an electrical engineering profession, but my work is completely different from electrical installation in a house. I took about 1.5 weeks with a colleague at our place. I borrowed machines from a buddy. Additionally, about 7-10 days after work where I did small stuff.
I can’t really say much about savings. Actually, I didn’t save anything at all. Because the money that was left over was spent on additional sockets, LAN ports, etc. It’s just a huge difference whether I pay 20-30€ net for a socket or only 5€ and a few meters of cable.