SQL Query to perform a "reverse exclusion" on a LEFT JOIN with a select from the left side table
You already have the table2_vals you need.
Just cross join them with table1. No need for long nested stuff and group by:
select t1x.Table1_Id,q.Table2_Val
from
    Table1 t1x
    cross join
    (
        SELECT t2.Table2_Val 
        FROM Table2 t2 
        LEFT JOIN Table1 t1 ON t2.Table2_ID = t1.Table1_ID 
        WHERE t1.Table1_ID IS NULL
    )q