ᐅ Insulation for radiator recess

Hello, I bought a terraced house built in 1969, which still has the old thick radiators installed. I want to replace them with 21mm (about 0.8 inches) deep flat panel radiators.
I plan to insulate the recess, which is about 18cm (7 inches) deep, but I’m not quite sure how to do it properly.
On the outside, the walls are made of red brick with no insulation in between.

My idea is to glue Styrodur (extruded polystyrene) boards on the wall using flexible adhesive, then cover them with an MDF panel, followed by a drywall (gypsum board), and finally mount the flat panel radiator on top.

Is this a suitable method, or are Styrodur boards not appropriate here? Above the radiators, there are windows under the window sill, and these will be replaced with triple-glazed units instead of double-glazed ones.

I would appreciate any advice or improvement suggestions!

Regards,
Waldi

Internal insulation requires careful consideration due to the shift of the dew point. Moisture may well (or likely?) condense behind your drywall, causing mold to develop. You should definitely consult a professional.

I agree with the previous poster!

Our neighbor ended up with serious mold growth because of this. I also recommend consulting a professional when it comes to triple glazing. Windows and walls should be compatible, otherwise moisture can condense on the walls.