Very thin screed, earth underneath - what to do?

In an old building (part of a farmhouse), the rear compartment is being renovated. After some chipping especially in the bathroom for the pipes, one can see that there is about 8 cm thick screed. (Apparently it consists of 3 layers, since these break up layer by layer during chipping. Twice about 4 cm layers, apparently made of the same material, and then a top 1 cm thick layer – presumably a leveling compound)

Since it would be quite a lot of work to remove the floor to dig deeper and install insulation, this is being waived. Applying insulation and then another screed on the existing floor would allow the room height (3m), but is also being omitted due to the effort. Since with around 30 ha of forest we do not have to save on wood chips or firewood, the possibly higher cost factor in heating is also irrelevant.
That’s my consideration so far.

In the bathroom (7 m²), the entire floor is removed and newly done, since too much was chipped here. An electric underfloor heating is also to be installed under the tiles.
Here now are my actual questions: Is heating without insulation sensible? How should the floor for tiles be best constructed? Is a simple screed (how thick?) including an uncoupling mat sufficient?

In the other rooms, there are partially small uneven spots. Should the floor be ground here or should a leveling compound simply be applied?

In an old building (part of a farmhouse), the rear compartment is being renovated. After a lot was chiseled out, especially for the pipes in the bathroom, you can see that there is screed about 8cm thick. (Apparently it consists of 3 layers, since these break open layer by layer when chiseling. Twice about 4cm layers, apparently made of the same material, and then an upper 1 cm thick layer – presumably a leveling compound)

To me, that sounds more like rammed concrete, and screed in the modern sense would only be what you call the leveling compound.

Since it would be quite a lot of work to remove the floor in order to dig deeper and add insulation, this is being avoided. Applying insulation on the existing floor and then another screed would technically allow the room height (3m), but this is also being avoided due to the effort involved.

You already have an enormous amount of effort just to produce a reasonably decent botch job. Then do it properly. Especially when the ceiling height permits it. Where is the additional great effort?

You already have an enormous effort just to get a reasonably decent botch job done. Then do it properly. Especially when the ceiling height allows it. Where is the additional major effort?

Well, strictly speaking, the botch job already exists. How should the new screed be installed on the rammed concrete then? Floating? As thick as a normal screed? Which layers and what material?

Well, strictly speaking, the botch already exists.

You can't say that, because depending on the year the house was built, that’s how it was done "back then," but from today’s perspective, you can only call it botch if you knowingly leave it like that while you are in the process of renovating.

Above all, the moisture barrier is missing. This is not good for the building fabric with normal heating behavior. And depending on the region, such a construction is extremely cold underfoot. You can hardly compensate by heating more. You could at most lay tiles, which would make the floor even colder underfoot. Electric underfloor heating is also pointless without insulation.