Solution
Converting variant data to a readable format can be difficult because it contains both the actual data and metadata about the type and format of the data. Therefore, there is no single function to convert variant data to a readable string. You have to convert the variant back to its original type using the Variant to Data function, and use string conversion functions, such as Number/String Conversion and Path/Array/String Conversion to get the correctly formatted string, which will be written to your file.
You can refer to the Data Type Parsing VIs palette to parse the variant data appropriately. With those functions you can obtain the metadata of the Variant, and use it to properly define the target type the variant is going to be converted to.