Here's another way:

DROP TABLE IF EXISTS #Products;

CREATE TABLE #Products
(
  Product VARCHAR(50) PRIMARY KEY CLUSTERED
, Type1 VARCHAR(50)
, Type2 VARCHAR(50)
, Type3 VARCHAR(50)
, Type4 VARCHAR(50)
);

INSERT #Products

This gives the same accuracy as your first 'exact' example:

SELECT CAST(1000000000000.123456 AS DECIMAL(29,12)) * CAST(10000.123456 AS DECIMAL(29,12))

This code helped me determine the CASTs to...

This part of the query makes no sense and should be removed. If the match conditions are true, there is nothing to update.