Which smart home system to retrofit?

Do you happen to know Homee? Would that be a suitable good overarching unit?

Homee? They still exist? I thought they were dead.

Why? Can't you just set the temperature there and the underfloor heating regulates the flow by itself?

Because the reaction times of a surface heating system usually range from a few hours to days. Therefore, it makes no sense to fiddle with it, for example, when coming home from work and wanting it warmer... or if you’re leaving, it also makes no sense to lower the heating.

The result will be that you always sit in the cold because the heating can't keep up. If you go on vacation for a week or two, then it makes sense to turn down the heating... but don’t forget to reactivate it 2-3 days beforehand, which can of course be automated.

On regular days, when the house is permanently occupied, it is usually counterproductive to tweak the underfloor heating. You heat based on outdoor temperature control and basically don’t touch the underfloor heating.

I also find the statement implying that new buildings without wired BUS systems must be labeled as old buildings to be too narrow.

No one has mentioned anything about buses. Read through what is written above. Conventional wiring is outdated and belongs in the realm of old buildings. (But it is still happily installed in quasi-modern houses... o_O). Structured wiring is the better solution. However, this then has little to do with buses. It can be quickly adapted to any kind of control variant. (Not necessarily BUS, it can also be a PLC or something else entirely.)

Conversely, this means that with conventional wiring you are very limited and then have to resort to crutches (Wi-Fi switches).

Why? Can't you just set the temperature and let the underfloor heating regulate the flow by itself?

If you want an inefficient heating system, then yes. The ERRs only know the state ON or OFF and nothing in between. With the thermostats, you always fluctuate around your desired temperature. If it is too warm, the thermostat closes. When the screed has cooled down, and thus also the air, the thermostat opens again. After a few hours, the floor heats up more than it should until the temperature at the ERR is reached. Then it closes again. I have deactivated most of my single room controls and have set the flow correctly once and for all. It's a bit of work, but it was worth it.

as far as I know, homee still exists

What do you think of Homematic IP?
Sorry, but as I can see right now, I have no idea about smart home.

There are also other options besides a wired system to set up a contemporary smart home (radio, WiFi).

You mean there are ways to satisfy the playful urge. What has been suggested here so far is actually quite far from contemporary and smart home. But yes, as a kind of modern model railway, the switches for the wall socket work.

What do you think of Homematic IP

Homematic is a pretty decent solution, if you can implement it in your place.

what does a modern smart home look like in your opinion, which functions does it cover? what would that mainly cost?

why shouldn't I be able to retrofit Homematic IP, after all that's what it was developed for as far as I understand

why shouldn’t I be able to retrofit Homematic IP, that’s what it was developed for as far as I understand

In the 90s, central locking systems for motor vehicles were also developed, sold, and installed for retrofitting by the dozen. But not just anyone could do it. First of all, we lack information about how familiar you are with electrical work, wiring, and also safety regulations and standards. Electricity is deadly and should not be taken lightly. Furthermore, it is unknown what is wired where in your case. Do you have a neutral conductor everywhere or not? Are there series and/or two-way switches? etc.