Infrared heaters throughout the entire house?

I would be able and willing to build again under optimal conditions (very small house just for me)

An alternative would be the split air conditioning system, heating in winter with COP 4-5, and cooling in summer, and no underfloor heating and hot water with an electric boiler

... the heating energy is supplied by the photovoltaic system.
I unfortunately forgot how the domestic hot water is heated (I think either purely electrically or with additional solar thermal).
...

That sounds unrealistic to me for Germany. We currently have great values with photovoltaic, but our air-to-water heat pump is running in summer mode now and hardly needs any electricity.
In winter, when the air-to-water heat pump used to consume about 40 kWh per day, on bad days less than 5 kWh came from the roof. The remaining 30-35 kWh you then pay your electricity provider for every day.

So calculate carefully what heating with infrared using electricity from the grid would cost you and compare that with the costs of other heat generators. I can't really imagine that it will be cheap. At least you shouldn't rely too much on photovoltaic in winter.
Heating the domestic hot water electrically is also not a bargain.

We also have the dream of synergy between photovoltaic and the rest of the building technology. However, we hope more for the air conditioning. When you switch it on, the sun will surely shine on the roof as well. :) The air-to-water heat pump only enjoys that during the transition months when you are already/still heating, but the sun is shining sometimes as well. Deep winter is rather disappointing for photovoltaic.

From my point of view, besides the financial and technical aspects themselves, the "feel-good factor" should not be underestimated! As 1-2 hints have already been mentioned here, not everyone finds infrared heat pleasant by far. I myself had the infrared heater in only one room for a few years and found it unpleasant and disturbing. In that case, the room was not used continuously or only for a short time; personally, I would not want it as the main heat source, even if it were only half as expensive to maintain.

I had a consultation last week with a prefab house provider. Of course, the desired energy class of the house was also discussed. KfW55 is actually already standard for a prefab house, KfW40 is also quite easy to achieve. Clearly, the houses basically only consist of a few wooden beams and insulation material.

Since I want to put photovoltaic panels on the roof anyway, you quickly get to KfW40+. My idea was therefore to go towards KfW40+ with a solar system, air-water heat pump, and underfloor heating.

A champagne cooler "belongs" in a Maybach – you can probably install it in a Dacia Logan too, but there it’s already quite far "off standard." KfW40 is a "feature" for a homeowner target group that is already somewhat established. Regarding the basics for everyday life, the checklist is already ticked off satisfactorily; now come the luxury extras like switching to organic, vegan, or the like. Solar and/or photovoltaic may be worthwhile, but it is something for people who can already think in terms of investing. A house in the category "space wonder & co" targets young families – they do not invest, they consume (and that includes the most basic necessities like baby diapers and the like).

Either a young family house "fits" you, OR you can already deal with avant-garde technology. I cannot and do not want to forbid you in a free country to mix these two worlds – but I want to give you food for thought that with avant-garde technology in a young family house, you create an atypical overall product. I see two consequences from this: in resale, you will have to fully write off the extra costs for the non-standard features; and during construction, you create details unfamiliar to the contractors, which usually lead to complications. The champagne cooler is not even provided for in the wiring harness of the Dacia Logan, and you also won’t find houses in the "space wonder & co" category as a Smart Home edition. If the moms need the closet space for drool bibs, there is no room left for disco mouse high heels ;-)

Therefore, I think: with KfW40+ and photovoltaic and infrared heating, you are in the wrong film in the range of house base models currently considered by you, and you should clarify for yourself whether the house class OR the equipment line fits you – both at the same time would be at least very, very unusual. If a house salesperson even considers such a "cross-build-kits" combination with you, it is explained by the fact that this area swarms with types who, to put it politely, are still very much in the orientation phase professionally. Unfortunately, beginners and career changers are much more common there than old hands.

Those who want real comfort are on the wrong track with infrared and heat pumps. Both are electric heating systems with major limitations. Pellets or gas is much better there :)

Here there is a lot of generalization again. o.O
We are also a young family, have still "invested" in photovoltaics and do not find that extraordinary as a feature for our 150m² place. We also have space for bibs AND high heels in the house, storage is a matter of organizational talent, not of m².

I would also not generally say that an air-water heat pump is an "electric heating system with major limitations" and that gas is generally better. We have been through the first winter and did not feel restricted in any way, rather the opposite. It was always a cozy warm 24°C in the house. That all ran via the inverter, we did not need the heating element. :)