Hello I actually wanted to do without external wall insulation, as opinions on the cost/benefit ratio vary greatly.
I had the same problem, only my conditions were probably somewhat better. My house dates from the 1980s and already has thermal insulation as well as a ventilated facing brick facade. Therefore, I limited myself to the critical weak points (radiator niches and roller shutter boxes) and insulated them accordingly; if necessary, one could later consider blown-in insulation between the masonry and the bricks. However, the cost/benefit factor is still unclear to me, especially since it would only be 4-5 cm. I already had a condensing boiler in the house and underfloor heating in most rooms. Radiators are additionally installed, but I hardly use them and they have a separate control circuit. Therefore, I will first monitor consumption over a few years to have concrete values for calculations. Your house is from the 1950s; I suspect that facade insulation is probably not present. Therefore, I might consider it again. For me, the problem was that the brick facade makes renovation rather more complex. But if you have a plaster facade, it might be worth it.