Single-pipe heating in niches. Relocate, insulate, or renew?

That doesn't really make sense to me, but he will know better ...

Insulation, like it was done in the past anyway, so a Styrofoam wallpaper with aluminum foil or something like that, where you presumably don't have to change the suspension and piping, doesn't seem effective to me at all.
But I'm going to withdraw from the discussion now. I do have my opinion, but not really experience ...

The room temperature in the apartment is still about 22-23°C, even now that the heating is completely turned off due to renovation work.

In Germany, it is winter in December (???)

The room temperature in the apartment is still about 22-23°C even now, when the heating is completely turned off due to renovation work.

That makes me curious how that works! If that is the case (which I doubt based on my own apartment wrapped in ETICS and triple-glazed windows, surrounded on three sides by other apartments), the ideal option would be simply to remove the heating and close the niches. Why heat if it is already too warm anyway. At that temperature, no mold should develop anywhere from ventilation, because it is too warm.

The problem with adding on is that it involves a single-pipe heating system, and one installer said that allegedly it does not work.

Find a professional ;-) P.S. Why not discuss insulating the building in the ETV once? Then you can also apply for KfW funding (grant option).

The heat in the apartment probably comes from the riser pipes running in the middle, as it is even warmer in the bathroom where the riser pipes run.