So how does it go in that equation, that you need to wait for 12 years until you have the nuclear plant built, with having 0 fossil energy replaced until then?

In the meantime you can scale Solar and Wind, where you continously can build it up. Also at much smaller costs, so you can build three times the raw power, that you could with the nuclear plant. Also you create an incentive, to not just think in terms of creating a supply that meets the demand, but also to adjust the demand to the supply, which is perfectly possible and a hardly tapped potential. Mostly because it requires the demand side to stop being comfy and actually improve their energy usage.

I agree, that we need to be effective. And it is more effective to have replaced much more energy supply in a shorter timeframe, while also getting the demand to adjust, than to wait around for 12 years, until then being able to replace the equivalent in oil or coal plants.