ALTER TABLE [YourSchema].[YourTable]
ADD Supplier AS CAST(SUBSTRING([YourStringColumn], CHARINDEX('-', [YourStringColumn]) + 1, 5) AS CHAR(5)) PERSISTED;

Now you can index the [Supplier] field, and join on it directly.

This should give you the the AVG per day as well as the AVG for the full result set.
SELECT DISTINCT
  isdfv.TotalTimeMeasure
, idv.Id

Combine your 2 statements like this ....
SELECT a = LEFT(a, LEN(a) - CHARINDEX('_',REVERSE(a)))
FROM (
  SELECT
   REPLACE
   (REPLACE
   (REPLACE
   (REPLACE
  ...

Change the CREATE FUNCTION to ALTER FUNCTION as you are now altering an existing function

You can create a function to calculate the durations for you.

The function below will calculate the time in seconds, using Mon-Fri 06h00 to 18h00.
However, it will not...

Comparison on NVARCHAR(MAX) is never going to give you good performance.

There was no need to move the data.

Option1 - Insert unique values into the original table
CREATE CLUSTERED INDEX CI_Temp ON exportLogTable2(TableID);

WITH cteData as...

When looking up the month names, you could also use the CHOOSE function

Replace this large CASE statement
case c.m when 1 then 'January'