Assemble fireplace kit yourself via the Internet

Well, if you light a 7KW fireplace in a new building, you'll have a sauna in no time anyway. It's only cozy in theory.

You mean uncomfortable in theory. Sure, the temperature in the installation room is much higher than anywhere else, but the heat spreads out in the open design. Besides, the fireplace isn't lit and then fired up at full power like a locomotive.

Well, if you light a 7KW stove in a new building, you'll have a sauna in no time anyway.

That's not quite right. We had 11 or 13 KW (see-through stove) in an approx. 75 sqm living/dining/kitchen area. Full power was enough for a 2.5-3 degree increase. But really only the combustion chamber and around it were beautifully built with drywall.

In theory uncomfortable, you mean. Sure, the temperature in the installation room is considerably higher than elsewhere, but the heat spreads out in the open design. Also, the stove is not lit and then fueled at full power like a locomotive

we have a Justus Reno R, so 6KW. We have 75m² living/cooking/dining as an open area, and then an open staircase connected via double doors to the upper floor. KfW40+. If we start at -5 degrees outside temperature with 21 degrees room temperature, after the first burn with three 30cm logs we have a comfortable 24 degrees after 1-2 hours, and after a second burn with again 3–4 logs nice 27 degrees in the living room and 25-26 in the kitchen. Upstairs barely changes, in the hallway yes, in the rooms branching off the hallway, no.

Therefore, it makes sense to use/have built a large masonry fireplace made of fireclay.
Fire it strongly twice – the stones slowly warm up and then emit consistent heat for a long time. In our 65m², the temperature rises by about 2 degrees.
With a stove like the one has, the full power immediately goes into the room.
With a stove like the one built, the heat goes up the chimney.

That is why it makes sense to use/build a large masonry fireplace made of fireclay.
Fire it up strongly twice – the stones slowly warm up and then radiate steady heat for a long time. In our 65m² space, the temperature rises by about 2 degrees this way.
With a stove like the one has, the full power goes into the room immediately.
With a stove like the one built, the heat goes out the chimney.

The stove weighs 230KG. That is, among the wood-burning stoves, rather one of the better ones.

I don't want to say anything against your stove, due to construction reasons it simply can't be done differently. Chamotte can (simply put) store heat well. About 800kg of chamotte were installed in mine; it "absorbs" the heat and then releases it evenly.