How can I use File.AppendAllText and add to it a new line?

public static void Save(string saveString)
    {
        File.AppendAllText(SAVE_FOLDER + "/" + "savegame.txt", saveString);
    }

The method is being called twice or even more times and each time with another saveString. The content of the saved file is like this :

{"objects":[{"Key":"367f6ac2-6fd3-4c99-91e5-cc335a104ac4","Value":"{\"s1\":false}"}],"instanceID":0,"position":{"x":8.140000343322754,"y":0.0,"z":0.0},"scaling":{"x":1.0,"y":1.0,"z":1.0},"rotation":{"x":0.0,"y":0.0,"z":0.0,"w":1.0}}{"objects":[{"Key":"ecb45f8e-463c-4fe3-a436-5d836165bcce","Value":"{\"s1\":false}"}],"instanceID":0,"position":{"x":-11.229999542236329,"y":0.0,"z":8.920000076293946},"scaling":{"x":1.0,"y":1.0,"z":1.0},"rotation":{"x":0.0,"y":0.0,"z":0.0,"w":1.0}}

And I want to format it to something nicer to be easier to look at maybe adding an empty line between each string ? Or some other format ?

This is how I'm saving it with json :

public void Save()
    {
        SaveObject saveObject = new SaveObject();
        for (int i = 0; i < objectsToSave.Count; i++)
        {
            var x = objectsToSave[i].GetComponents<Component>();
            var y = x.Where(component => component is IStateQuery).ToList();
            List<KeyToValue> myObjects = new List<KeyToValue>();
            foreach(var z in y)
            {
                var w = z as IStateQuery;
                myObjects.Add(new KeyToValue(w.UniqueId.ToString(), w.GetState()));
                
            }
            saveObject.position = objectsToSave[i].position;
            saveObject.scaling = objectsToSave[i].localScale;
            saveObject.rotation = objectsToSave[i].rotation;

            saveObject.objects = myObjects;


            string json = JsonUtility.ToJson(saveObject);

            SaveSystem.Save(json);
        }
    }

But I don't like the format of the content in the file.

I tried to add Environment.NewLine :

File.AppendAllText(SAVE_FOLDER + "/" + "savegame.txt" + Environment.NewLine, saveString);

But it's giving me error :

ArgumentException: Illegal characters in path.