I am writing an EF migration to update data in a JSON column, names are being simplified into a single field. As a first step I want to populate the new field. This is what I have come up with having looked at many SO questions and answers (copilot was little help)

DECLARE @t AS TABLE 
              (
                  id int IDENTITY(1, 1), 
                  details nvarchar(max)
              )

INSERT INTO @t (details) 
VALUES (
 N'{
    "Contacts": 
    [
        {"Id": 1, "FirstName": "John", "LastName": "Doe"}, 
        {"Id": 2, "FirstName": "Peter", "LastName": "Pan"}
    ]
}')

SELECT 
    -- will replace with UPDATE
    JSON_MODIFY(
        details,
        '$.Contacts',
        JSON_QUERY((
            SELECT '[' + STRING_AGG(updated_json, ',') + ']'
            FROM (
                SELECT JSON_MODIFY(
                    c.[value], 
                    '$.Name', 
                    CONCAT(ISNULL(NULLIF(JSON_VALUE(value, '$.LastName'), ''), ''), ', ', ISNULL(NULLIF(JSON_VALUE(value, '$.FirstName'), ''), ''))
                ) as updated_json
                FROM OPENJSON(Details, '$.Contacts') c
            ) t
        ))
    )
FROM @t

This returns the following output:

{   
    "Contacts":
    [
        {"Id": 1, "FirstName": "John", "LastName": "Doe", "Name": "Doe, John"},
        {"Id": 2, "FirstName": "Peter", "LastName": "Pan", "Name": "Pan, Peter"}
    ]
}

Is there a better way to write this without the string manipulation adding the [ and ] with STRING_AGG?

All my attempts added a new node [{"updated_json":{"Id": ... e.g.

SELECT 
    -- this code returns the "updated_json" extra node
    JSON_MODIFY(
        details,
        '$.Contacts',
        JSON_QUERY(
            (SELECT 
                 JSON_MODIFY(value, '$.FirstName',
                             CONCAT(ISNULL(NULLIF(JSON_VALUE(value, '$.LastName'), ''), ''), ', ', ISNULL(NULLIF(JSON_VALUE(value, '$.FirstName'), ''), '')) 
                    ) as updated_json
             FROM OPENJSON(Details, '$.Contacts')
             FOR JSON PATH, WITHOUT_ARRAY_WRAPPER))
    )
FROM @t