In which season of the year is it best to bring screed into the basement

Well, thermal bridge? For that, you decouple the walls from the "cold" floor ;)

Yes, of course, but nevertheless the concrete slab is "cold" and this basically draws the walls up from there. But as I said, in theory you would have to insulate under the BP and also the walls, then it would be completely decoupled.

Yes, my (Innen)Wände are decoupled from the floor ;)

Yes, but the exterior walls are made of concrete and are therefore also thermally connected to the BP.

Correct, but the thermal bridge is then quite minimal again due to the external insulation ;). But yes, ideally there would be insulation underneath - personally, I don't support that for cost reasons and regarding the execution.