We are planning our smart home in the single-family house

Where did you get that nonsense from?
Obviously not from studying or practical experience.

Wrong thought.

And that is also wrong.
Reacts pretty quickly.

In old buildings you notice very quickly when the circuit is closed. Not in new buildings. But maybe you made your experiences in old buildings.

Here you apparently also have no practical experience.
My experience: works just as well as heating!
In winter, I have a delta T inside and outside of sometimes over 30 degrees when heating.
In summer, when cooling, very rarely more than 5 or 10, so inside 23, outside 30 is just about 7 degrees delta T.
That can be cooled away very well.

Own experience for years with air-water heat pump and cooling via underfloor heating. Cooling is only possible to a limited extent. The increased humidity is also still bad.

Haha, you are something else, first say it doesn’t work at all and has to be done with great effort and then say you just do it somehow and wonder why it doesn’t work?!

No, you should learn to read. Fully professional with sensors or pragmatic with empirical values and technical gadgets.

But honestly, what’s the point of the discussion?
How can anyone get the idea to make their house smart, spend tens of thousands of euros and then want to save a hundred for an actuator plus a few tens for actuators, because someone on the internet said that “underfloor heating adjusts itself”?

Well, if your advice for building a house is that it doesn’t matter and just buy unnecessary things anyway, then so be it. No wonder people always complain about how expensive houses are.
By the way, underfloor heating does not adjust itself. It is planned properly and does not have to be adjusted anymore (assuming hydraulic and thermal balancing before you start quibbling). By the way, this is not said by "someone", but is state of the art.

We have switchable sockets, but in groups. For example, all sockets for Sonos speakers on the ground floor, a socket group for entertainment in the living room... This way, unnecessary consumers can be switched off when absent/asleep and save electricity. In my opinion, this is very convenient and sensible.

Yeah, you can definitely construct use cases to somewhat justify the investment.
I did that at the beginning as well. Calling it convenient is rather "interesting." It's more like super inconvenient when you want to use such a device and have to wait until it has started up, connected to the network, and then is usable.
Turning off the socket for the OLED is also a very bad idea. It won’t handle that for long...

So what remains is maybe saving 2W of power consumption. So about 2€ per year. And that with a KNX system that cost a few thousand euros.
Can’t convince me. ;-)

No OLED in the household, so it doesn't matter...

Everyone is always raving about KNX and SmartHome, but when asked, no one could concretely name a few use cases that make sense. At least you have already created a nice list. Many things are nice to have, but I don’t see a real benefit. With LEDs, it doesn’t pay off anymore to constantly turn the lights on and off in all rooms. Voice control almost always takes longer than pressing a switch. For some of your listed points, KNX is not required; the products bring the function directly with them.

Here are a few remarks from me:

(2) In the guest bathroom, music should start playing when presence is detected, and possibly the ventilation should increase a bit

Hmm, ventilation turning on with the light has been around since the 90s or longer. What does that have to do with SmartHome? The music part is cool. You probably won’t notice it yourself after a while, but you can impress guests with it.

(9) We want to avoid switches wherever possible; we prefer to control by voice or ideally completely without interaction (automated)

I would never do that. What do you do if you are sick and can’t or don’t want to speak? What about guests who don’t know the commands? What about children who can’t pronounce the commands properly yet? At a certain noise level, it will be impossible to turn the light on or off that way. Not to mention that it takes much longer than simply pressing a switch.

(2) CO2 sensors, for example in the study, to control ventilation depending on presence/absence (ventilation for example on max when I briefly go to get coffee and the CO2 value is above X?)

This is probably about the noise of the ventilation, right? Your SmartHome can’t know if you leave the room for 30 seconds or 10 minutes. And if you don’t leave at all, does the air stay bad? That makes no sense. Better just a sensor that triggers a notification when a certain value is reached. Then you can decide yourself whether to open the windows or turn on the ventilation (by voice).

(3) Networked smoke detectors as an alarm system + to hear a fire alarm from the ground floor in the bedroom (?)

Networked smoke detectors exist from pretty much every manufacturer and usually make sense in a single-family house. That has nothing to do with automation.

(4) Vacuum robot in the hallway that starts automatically when the house is entered and/or exited (dirt in the hallway)

That won’t work in practice. Or is it supposed to start every time you have visitors or one of you just goes briefly to the garbage bin?

(5) Lawn mower that only runs in certain weather + when no one is in the garden

Weather and time should not be a problem. You don’t stay in the garden around the clock anyway, so you just set a time and that’s it.

(6) Automatic garden irrigation depending on the forecasted weather (cistern planned)

Equipping the irrigation with sensors basically makes sense.

(7) Hand towel heater in the bathroom to be switched on/off locally (e.g. warm bathroom in the morning, warm towels right after showering)

Do your days always run the same way? Ours don’t. There are holidays, sometimes there’s a shower after sports, or sometimes before going to bed in the evening. Accordingly, it would have to be switched on almost all the time.

(8) "Speaker announcement" from the kitchen to all children’s rooms ("Dinner is ready")

We do that with Alexa.

Hmm, ventilation on with light has existed since the 90s or even longer. What does that have to do with SmartHome?

Try to sensibly link the light switch with the controlled residential ventilation system without Smart Home, then you will see.

It's probably about the noise of the ventilation, right? Your SmartHome can't know whether you leave the room for 30 seconds or 10 minutes. And if you don't leave it at all, the air stays bad? That doesn't make sense. Then just a sensor that gives a notification at a certain value. Then you can decide whether to open the windows or turn on the ventilation (by voice).

Smart Home of course also means smart programming – for example, you can link two sensor outputs via logic, such as VOC + presence, to trigger different scenarios. For example, to react differently when "presence detected" & "VOC out of range" -> controlled residential ventilation at level 2 of 4, or "presence not detected" & "VOC out of range" -> controlled residential ventilation at level 4 of 4.

Whether this makes sense here, we’ll leave aside for now.

Do your days run the same? Ours don’t. There are holidays, sometimes people shower after sports or in the evening before going to bed. Therefore, it would have to be on almost all the time.

A sensor with humidity measurement can detect relatively reliably when someone showers (even more reliably, of course, with a flow sensor on the corresponding pipes). Then, for example, a switching actuator for the radiator can be activated automatically, except if, for example, the weather station reports over 15 degrees outside temperature and/or the window contact sensor reports the windows as open/tilted.

As you can see, it only becomes smart when you can measure the various states sensibly and link them sensibly.

That will not work in practice. Or is it supposed to start driving every time you receive a visitor or one of you just briefly goes to the trash bin?

For me, this is actually one of the "coolest" functions. We have a relatively long hallway with several meters to the coat rack. If it is regularly vacuumed and mopped here, it can prevent dirt from spreading further inside the house. This can also mean vacuuming after taking out the trash. Since the area is relatively small, the vacuum/mop would be done again after 5 minutes. However, the function could also be linked to a button at the exit or the voice command "Alexa, bye" so that the robot does not start driving at every little thing. You have to test what fits best into the routine.