How to select a Taxonomy term in a Pane's settings and send it as an argument to Views?
Maybe my initial request has a valid answer, but it is easier to forget about the contextual filter and only keep a relationship Content: Taxonomy terms in node to select the correct vocabulary. Next, filter on Taxonomy Term: Term and select the previously added relationship. Now you can select the required term from a list! Of course, you can duplicate the Views block as many times as needed and only override the taxonomy term. Finally, these blocks can be used in a Panel or elsewhere.