Currently, there isn't a way to assign a due date per group or schedule courses. I'll submit those as feature requests, though.
Since the due date is set at the course level, you could have two versions of the course, one with a due date or by days and another without. If the courses absolutely need to be assigned in different weeks, then you would need to go in and assign the courses to those learners.
But depending on your needs, you might be able to create a learning path with all of the onboarding courses and require that the learners complete them in order. I would then stagger the due dates for those courses (for example, the first week would be 7 days, the second could be 14, and so on). I hope that helps. Please let us know if you have any questions.