Would be good if you could get the categories in Outlook Tasks to Flow by creating new Plans in Planner with all the Category names appearing as new plans. And then automatically populate the new plans with all tasks in Outlook Tasks under those categories. It would be better still if Planner simply synced Outlook Tasks to Planner creating new plans for each category and putting the tasks under them. And had a quick simple button on Planner to do this sync. Perhaps you could liaise with Planner engineers to do this or find out when they are doing it.