How to count tasks across all sheets in a workspace


I have a master metrics sheet that I am using. I'd like to be able to count tasks by assigned to across all sheets in a workspace. The goal is to have this formula work if/when any new sheet is added to the workspace.

Reports are not really working for me because my assigned to column is a multi-select column. I don't see a way to add a 'Workspace' as a source to a countif forumla.

The end goal will be able to build a metric or chart in a dashboard to display the data.