When I run your query with the date set to 21/05/20, the WEend Raised - Sund is the 24th and the Thursday one is the 21st. Does this not match your requirements?
The problem I see coming up while doing some testing is that if I change the date to the 22nd, I expect the THURSDAY call to be the 28th, not the 21st again, right? If so, that is because your DATEPART portion is coming back larger than 5 and as such, it is subtracting to get the Thursday of that particular week. If you want to get the following Thursday, you are going to need a CASE statement to ensure you don't go negative with the DATEPART. So something like this for your Thursday calculation:
,[WEnd Raised - Thur] = MAX((CONVERT(date, DATEADD(dd, (CASE WHEN DATEPART(dw,[DateTimeRaised]) > 5 THEN 12 ELSE 5 END) - DATEPART(dw,[DateTimeRaised]), [DateTimeRaised])
What your date conversions are doing IF the number minus (8 for Sunday and 5 for Thursday) is less than 8 is grabbing the day of the week for the current week. So the 5 is grabbing the THURSDAY of the week that starts with SUNDAY.
NOTE - the above assumes your weeks are starting on SUNDAY. If you adjust it so your weeks start on a different day, the calculations need to be shifted.