Which smart home system is suitable for our new building?

Sorry, but I have never heard such nonsense. Are you also not allowed to install lamps on the ceiling because of the statics? That is unfortunately nonsense as well. Wifi 6 and good APs can't do anything against poor positions. Radio works similarly to light. So where light doesn't get through, usually no WLAN gets through either. That's why lamps are also installed on the ceiling and not behind the TV.

In principle, you are right, but more antennas (and XxX Multi Mimo) in a better access point can still be sufficient for the OP even in a bad position. If he absolutely doesn't want to do it otherwise? ... I'm out of this thread now as well ...

What does central management mean? What exactly do you have to manage in a single-family house?

What does your network cabling generally look like? Who does it and how? Somehow I strongly suspect that you have something like homeway, where you (hopefully in all rooms) have a socket with network and TV connection and a box in the HAR where you are supposed to plug in your router. If that’s the case: Just take a FritzBox or whatever your internet provider wants to give you, that will be enough. One access point per floor will be sufficient for your floor plan. Just take a repeater and plug it into a socket (not hidden behind anything!), or lay an extension cable onto a cabinet and place the device there. If you have network sockets in all rooms, connect the repeater via Ethernet cable ("LAN"), otherwise simply use it as a Wi-Fi mesh network. From everything that has been said in this thread, that will be enough for you. You will not be managing a network with VLANs and your own firewall settings. I would also like to concretize my tip for the smart home system: Just take a look at HomematicIP, I think it will be the most suitable solution for you. Get a WLAN video doorbell that you can link with your Echo Show and you’re all set.

Thank you for your detailed response.

Yes, we will install a LAN double socket in every room (except in the dressing room and the bedroom, we are still considering) and one double socket in the garage.
Each LAN socket will be accompanied by an antenna cable (2 in the living room).

Antenna cables and LAN cables will of course be distributed in a star topology from the technical room in the basement to all rooms.

The antenna cables go to the multiswitch from the satellite dish and the LAN cables to the patch panel.
The electrician will take care of that.

Homematic IP is definitely an option (which covers almost everything).

I am better acquainted with software (and can get more involved in it) than with assembling the right hardware.

In our rented apartment, the standard router from the provider stands in the storage room and we manage fine with the Wi-Fi.

Most of it about KNX has already been written here. However, finally from me two tips and a remark:

There is a good and very extensive "KNX User Forum." Just google it. You can exchange a lot about the topic of KNX there. Sometimes people go very far with their SmartHome installation.

There is a very thick book on the subject. Possibly suitable for further details and deepening.

My personal opinion on presence detectors/motion detectors: I don’t want to be a slave to my house. My house will never know 100% if and how I want my light. Therefore, motion detectors only in certain rooms where it makes a lot of sense (e.g. utility room), and in the other rooms there is an early cable in the ceiling for future ideas.

Just to be clear, we’re still talking about equipping and living in a 150-180m² single-family house with a laptop and TV, right? Or is it now about running a highly available industrial production with CAD, CAM, CAQ and everything else, or equipping a hotel?

Here in my 110m² rental apartment with brick walls, I have 2 mesh routers standing around, I just put them there without any analysis of radiation angles, etc. And I get nearly 250 Mbps everywhere on the Wi-Fi...

I think motion sensors are great for the stairs. I don’t have to leave the light on all night and the kids don’t fall down because they forget to turn on the light half asleep. Also, for the outdoor lights I think it’s brilliant, ideally integrated directly in the lamp.

And with the builder, it just is what it is. That’s because they usually offer houses calculated as finished, where the structural engineering doesn’t need to be touched much anymore. You can think that’s stupid, but that’s how it becomes cheaper...

I also use a mix of different systems (zigbee, WLAN). As a central unit, I have set up an ioBroker running in a VM via Proxmox. This has been working very well for >1.5 years. For WLAN, I use Shelly's. I know that this is not the optimum in terms of "long-term stability," but I had to replace all the previous enocean actuators because the range was too short and, for example, the blinds could not be controlled to specific %-open positions. That is just one example from "practice." Bus system via cable was unfortunately not possible for us for various reasons.

I also believe that there is no shame in relying solely on a wireless system. I think there are certainly KNX houses that also have wireless systems integrated.

If I ever build a second house, I would definitely lay bus cables / wire it in a star topology. And I would not let a sub-electrician just do the standard 08/15 electrical work. But as described above: it also works with wireless. However, the long-term test is still pending.