Is a fireplace still useful in new buildings today - experiences?

You can only consider the stones as a buffer. If you put too much wood in and it gets too warm, the buffer has a negative effect because you release too much heat into the room over a long period of time.
7-8kw is already quite substantial, especially in a new house... I could imagine that it's too much of a good thing!

My stove builder, if I remember correctly, recommended a 5kw stove for a renovated old building with a 55m² room.
A stove that's too big and loading too little only works to a limited extent; a smaller stove and too much wood also only works to a limited extent.
In the 55m² room, I usually don’t set the heater above 3 in winter (no underfloor heating). That's of course a relative figure, but the room is still always 20°-22°C.
I suspect that with a wood-burning stove I would rather regularly overheat the room.
I can only advise you to have everything planned carefully; installing on a guess can be an expensive mistake.
If it’s "only" about the fire and the crackling of wood and heating is secondary, you can also take a small stove and enjoy it. That way you at least avoid the risk of overheating the room.
In any case, if I still retrofit, I would rather take too small than too big.

In a new building, a wood stove in the classic sense is usually not economical regarding heat and energy costs.

A pellet stove can be easily downregulated and, with a water jacket, some only release 10-20% of the heat directly into the room. This results in only about 0.3-0.5 kW going directly into the room.

It is not about wanting the stove to pay for itself in that sense. In the rental apartment, we have the package you suggested and unfortunately, we don’t like it at all. We light the stove, but it only gives off 10% heat into the room; everything goes into heating the water. I do want to use the stove for the transitional period when the heat pump then rather tempers the house instead of heating it up and does so very slowly, so that I can quickly warm it up if I feel like it. But I don’t want to have to sit there in my underwear… I had it exactly planned by two stove builders… but everyone suggests something different, see above.

So, I have a masonry stove with a water jacket AND ceramic flues (7m) where I can switch between the water jacket and the flues. The combustion chamber is designed for up to 10kg of wood, and depending on how I burn (water jacket or ceramic), more or less heat ends up in the room. But about 20-25% of the heat definitely comes into the room through the glass.

My stove stands as a room divider between the living room and dining room/kitchen. So far, it has never overheated the room. It takes about 2 hours for the surface of the stove (about 4sqm, I guess) to reach 45 degrees. After 24 hours, it still has 20-25 degrees. When it is completely cold, it takes about 20-30 minutes until it radiates heat through the glass. After an hour (one burn with 10kg of wood lasts about 2-2.5 hours), you notice that the surface (fireclay bricks of the outer shell) gets warm. If I just want to 'temper', I only do one burn. After that, I close the flaps, then the heat slowly presses from the inner shell to the outer shell.

I think it’s brilliant. Especially if you add more wood in the evening (around 8 pm) in winter, then the next morning you have a nice lukewarm stove warming your back!