Judging from my experiences and other tutorials, including the one from Epic Games, it seems normal that you are not able to edit the default values in the editor. I guess I will just have to use the .cpp file for that kinda stuff.
Example: Introduction to Programming | Live Training | Unreal Engine - YouTube
When he creates the blueprint, you can see that the “rotation rate” variable is also greyisch like my variables, meaning he cant edit them either.
Edit -
Nevermind, this got it to work somehow:
UPROPERTY(EditDefaultsOnly, BlueprintReadWrite, Category = Health)
Jokes on me.