Base plate with concrete core activation. What is your opinion?

No sources, plain engineering sense. You can take the risk, but at the latest when the tiler says he cannot lay tiles here, you have a problem. I wouldn’t risk it and would always plan with an additional screed. It doesn’t cost anything either, so why even waste a thought on it? You yourself write at the beginning that you hardly find anything, or have you found a construction blog with [Schwedenplatte] and solid construction?

So seriously, I think that's a guess. You don't know it, so it carries a high risk. I think the panel would be off the market if there were regular problems. Otherwise, I would just call and ask. And in the end, I would try to contractually bind the company to a tolerance. If that is rejected, you can still replan. I like the principle. All the mass is within the thermal envelope. Especially the concrete core activation contributes to low supply temperatures, which costs real money with heat pumps.

...does not cost... but saves...

You do read online, however, as a counterpoint, that the surface is not always as even as you would need it for a flooring. Then it is sanded down or leveled, but nobody calculates that in advance. Whether that is true, I do not know. With such statements online you always have to be cautious. There are usually very positive as well as negative statements about building materials (depending on the author and their motivation).

I don’t have any concrete offers now, but according to my research the cost difference between conventional and thermally activated concrete floor slabs is not very large.

Well okay, but what is this about anyway? Functionally, the Swedish slab in my opinion has no real advantage, so it’s about the price.

a difference of €200 to €1,000

I haven’t checked the numbers to see if that adds up. But I consider the difference significant. We are calculating about €110 per sqm for the slab, including the necessary insulation (KFW 55). That means, for example, an 11x11 sqm "city villa" €13,310. Your €800 price difference corresponds to 6% additional cost. If you keep that attitude of "it doesn’t cost anything," you can calculate for your whole project what that means in money.

No sources, just plain engineering common sense.

That is probably one of the most impressive remote diagnoses I have ever read here. Where do you buy your crystal balls?

The structural engineer will calculate that.

At an acquaintance's place, plastering was also done on the screed. Vinyl was glued. Isn't that usual? The screed layer doesn't achieve 2mm over 1 meter either, does it? I simply lack the experience.

Very funny statement, even the structural engineer is bound by the limits of physics...