I believe that you're looking for functionality given by precedence constraints. These are the arrows that define the order of the tasks (you can double click on them).

Something like this?
I included an option with an inline table-valued function and one direct option. The function won't have a performance impact.

CREATE FUNCTION dbo.YearsDifference(...

Here's an option. Make sure that you understand this and ask any questions that you might have. You can find the code for the function in this article.

This works for your sample data. Make sure it works for more data and different scenarios.
The CROSS APPLY is there to prevent repeating the same condition several times.

The suggestion about AdventureWorks was to help you design your database. Currently, your problem is on the design, not the code.

This might help: https://www.sqlservercentral.com/Forums/Topic751783-566-1.aspx

What I mean is that the usage of an index is only possible when you search for the beginning of a string. Check some examples in here: http://www.sqlservercentral.com/articles/T-SQL/130558/...

No index will help for this. You'll probably want to search inside the strings, so that would make the queries non-SARGable. Full Text Search would work great for the CompleteDescription...

I'd prefer if you stay out of dynamic SQL for this. Here's another option.
SELECT CASE MONTH(GETDATE())
   WHEN 1 THEN JanTarget
   WHEN 2 THEN...