The best way I’ve found is to create a “Completed Tasks” section and create a rule that automatically moves tasks to this section. Then, when I’m ready to “batch delete,” I delete the “Completed Tasks” section, select the option that says “Delete Section AND Delete ### tasks?” After that, I simply create a new “Completed Tasks” section and reconnect the rule. It’s not as great as “Delete All Completed Tasks Right Now Because I said So” but it works.