How to create multi select filter for filtering a CTE output

I have a SQL query that has multiple CTEs and then those CTEs are combined to form the final output. The issue I am facing is that I cannot use a Field Filter to create a multi select filter here since the filtering is happening on the custom fields created within one of these CTEs. The final output of the values in this CTE has no reference to an actual value in my table columns being queried here and are created by using CASE conditional logic on multiple column values to get this output.

Is there a. way to enable multi select for such a field? I am also curious as to why Metabase doesn't allow any other filter type such as text, number to have multi select values when a lot of other tools seem to be able to accomplish it?

Why don't you move all that logic to a Database view? so metabase would see this view as a table and allow you to work with it as if it was a table including field filters and so on