“First business day” is going to be hard to define. But I also have a task that recurs on a monthly basis. “Every Month on the first Monday” works just fine (which is what I needed).
This may be a silly question - but why can’t you just set it to recur on the 1st and 15th? What is the worst that will happen? You’ll come to work on Monday and be greeted by a todo list item that populated over the weekend? Depending on your needs, this may be a problem that doesn’t require a solution.