The trigger records all right when I am creating the macro and it stops the default action of iTunes from happening, but it doesn't actually execute the action!
Any pointers or debug steps welcome.
If a macro does nothing, the first thing to try is the Help ➤ Assistance window. Select Something expected is not happening and go from there. That will tell you if the macro is being triggered or not.
If it is not being triggered, then you need to look at the activation or triggering.
If it is being triggered, then you need to look at the actions.
But either way you'll have a lot more clarity.
Yup, got close — I can bind any of the volume buttons, as well —
But when I get to the Assistance window, I am stuck on the last step "Waiting for the Macro to Trigger…" and none of the events seem to get "caught". For volume change macOS still changes the volume, but for play/pause button, it seems macOS default of "start iTunes" is no longer being triggered, either (so that's nice!).