Somthing like /exnihilo reload

Such a command would end up being limited, the ore registry wouldn't be reloadable (since the minecraft item registries aren't) nor would JEI be updated for the new recipes.

So there is this command in the code base /enreloadconfig. Does that do what you want?

It reloads config not jsons
EDIT: I'm actually stupid it DOES reload json but JSONs errored.
It's good then