I have a partly solution for boards.

When I created a gitlab board that should track all issues with label X and also either Label A1 or Label A2, I could get this filtering work like so.

In the Issue Boards search bar I added the filter for label X
than I clicked on 'configure Boards' -> Labels (Any label) and added here Label A1 and A2

The resulting board displayed all issues with label X and either label A1 or label A2 attached.