Add 3 levels of task IMPORTANCE, easily visible by brightness/shade of the task tile background
In addition to tags (which are most useful to assign categories to tasks, i.e. #HOME, #HOBBY etc.), please give the users an ability to mark the IMPORTANCE of tasks. For the sake of not overcomplicating this, it will be enough if you enable the user to pick one of the three IMPORTANCE categories: HIGH or MEDIUM or NORMAL (normal being default). In the dark theme, the HIGH importance tasks would have the brightest background, the MEDIUM ones - a somewhat darker one. In the light theme, you could use a theme-dependent leading colour in 2 hues or give important tasks solid white background and the MEDIUM ones a somewhat more transparent white yet still standing out. Example attached!