Try adding this line ...
, AvgSeconds = AVG(IncidentStatusDurationFactvw.TotalTimeMeasure) OVER ()

I do not have access to your data, so anything that I post is simply an...

This should get you the desired results
SELECT
  src.ShipperID
, src.InvtID
, src.CONTAINER_NO
, src.ITEM_CODE
, NetWgt = SUM(src.NET_WEIGHT)

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.