Merge two dimensions/filters into one?

Hello,

I have five billing periods on my data source. Please see the original dataset:

I excluded first two for reporting on my dashboard (“BAK2017” and “BAK2018”), so I created a set of three (“Billing Period Set”) which I am now using as a filter:

However, the last billing period “UUBAK2020” should be merged and reported together with “BAK2020” (under the unified label “BAK2020”, so it captures total packages sent from both reporting periods-21).

Below is the result I am hoping to get, only two values presented in the filter BAK2020 and BAK2021 (also includes values from “UUBAK2021”).Can you please tell me how to merge two filters/dimensions into one (to get the result above)?

Thank you for your help