I read your question and it is possible to use Json or even XML but I am asking you this. Why extension and representation type is important ? Do you want to view it outside of engine such as a web project ?
Unreal Engine has its own blueprint exposed Json plugin. You can use it. Marketplace plugins are really cheap, you can use them.
But you can also use Unreal’s own save system. It just creates you .sav file.
And all cases whatever you want to save, you need a plan. Especially for Json, you need to plan your structure. Because plugins are like this.
SetStringField
SetBoolField
etc.
They are not like “record transformation” or “record actor”. So, you don’t need to focus on extension. It is not really that important.