In this case, why use the Task Queue? If all items need to be processed, I believe the use of a Task Queue may not be necessary.
However, you can include a validation inside the SUB_Microflow to check if all items have already been processed, and if so, trigger the "END" flow.
Use the Task Queue Manager from the marketplace: https://marketplace.mendix.com/link/component/220289
With this module you can check if all your tasks that where put in the queue are finished (do check also the status of the task).
So create a microflow that retrieves all the just created task and check their status if not use the Delay function of the community commons to wait X amount of time before checking again. Use a loop counter so you do not create an endless loop in your model but quit after a certain number of loops. If all the queues are correctly finished trigger the next flow to do the rest of your stuff.
Regards,
Ronald