Internal insulation because of mold stains?

Good day,

I have a question about my rental apartments.
In the rental rooms, at the spots circled in red in the picture, there are occasional mold stains with some tenants. So, there are several apartments in several buildings with this room layout; about 4 out of 12 currently have the problem.
The exterior wall is a 30 cm clay wall. Plastered from the outside and painted 5 years ago. No additional insulation.
As seen in the picture, the construction promotes the formation of mold stains because the window and door face each other and air cannot circulate in the back corners.
As far as I have read, the high humidity caused by sleeping, the warm air from heating, and the cold exterior wall cause condensation to form, which leads to mold stains.

Regarding my idea:

Would it help if I installed stud walls + insulation + vapor barrier + drywall on the inside of the exterior walls?
My hope is that the heating pipes (green in the picture) running between the exterior wall and the new stud wall create a warming insulation layer.
So that the formation of mold stains is minimized and heating costs are somewhat reduced. Due to the room conditions, I probably can never fully eliminate this if tenants ventilate and heat incorrectly. But I want to at least achieve that normal ventilation and heating is sufficient.

Complete insulation of the facade is currently not possible for cost reasons.

I know the topic from my own rental experience. We insulated from the inside with calcium silicate boards (specialist company). With drywall, you really take away a lot of space from the tenants – and whether it then also works (dew point shift) is another question. The success has been so-so so far, but definitely a clear improvement.

And what if you install windows there?

What did you do on the boards? According to the description of the boards, a woodchip wallpaper is not effective. Only glass fiber wallpapers or diffusion-open paints may be used. Does that mean I would also have to repaint or cover the interior walls?

Replacing the windows would be almost my last solution. The construction effort in an inhabited unit is already very high. That would only be considered in a complete renovation of the property.

A painter I know suggested using paint with fungicide.

What did you do on the panels? According to the description of the panels, a woodchip wallpaper is not goal-oriented. Only fiberglass wallpapers or diffusion-open coatings are allowed. Does that mean I would also have to repaint or cover the interior walls.

If, then renovate properly. Honestly, I have no idea what was applied to the insulation. I only approved the order through the property management. I think it was only painted.

Install decentralized ventilation system?

We are apparently planning that now in an apartment. Install 6 core drills, 6 devices + one control unit. Each device just needs to have a power connection. We will carry it out when the apartment is unoccupied, but floors will remain in place. Occupied would also be possible in an emergency, I think, if the core drilling is cleaned properly.

Cost approximately 3,200,-

We hope to get rid of the problem of mold stains in the corners of the house on the north side with this.

Regards
Specki