Multiple selections using data validation (sumifs)

Not sure what the best method is but I want to be able to select multiple criteria in column a. for example, United States and United Kingdom and then my sumifs includes BOTH of those countries. I could put another country field but the selections might be variable. Like I might want to see top 5 markets or EU and i don't want to just create the aggregate data for them on their own. Also if possible with the other criteria (e.g. class and/or account type).

Not sure if maybe adding a macro with a "+/-" button would work and if I add a + it will add those 3 additional fields and update the SUMIF to encompass that? What are your thoughts?

Revenue Metrics Dashboard.xlsx