Which of these SQL queries will be better for query performance?

Question

So I have some tables with millions of rows of data, and the current query I have is like the following:

WITH first_table AS
(
    SELECT 
        A.column1, A.column2, B.column1 AS column3, C.column1 AS column4 
    FROM
        tableA AS A 
    LEFT JOIN 
        tableB AS B ON A.id = B.id
    LEFT JOIN 
        tableC AS C ON A.id = C.id
    UNION ALL
    SELECT 
        D.column1, D.column2, NULL AS column3, D.column4 
    FROM
        tableD AS D
    UNION ALL
    ...
)
SELECT 
    column1, column2, column3, column4, A.col5, A.col6... until A.col20
FROM 
    first_table 
LEFT JOIN 
    tableA AS A ON first_table.id = A.id

I'm basically appending two tables at least to table A and then joining again table A in the final SELECT statement. I do this because I need like 30 columns from table A and I don't want to fill with NULL values the append statement since I only need 4 or 5 columns from the tables appended to the main one (tableA).

I was wondering if it would be better to avoid the join and then fill all the columns I need since the WITH statement or should I keep my code as it is. All of this is for query performance and improve execution time.

Thanks.