The thing is the barrier is not very clear between different series.
So what you might want to do is study both beginner and lower intermediate series at the same time. Both will bring you new knowledge; bot some of it will be a bit challenging, thus meeting your desire for challenge.
And definitely listening to the lessons below your normal level is a good idea. It helps to review concepts and vocabulary you already know, and on the side will bring you a feeling of: OK, this I can understand perfectly.
One day, a beginner lesson, the next day, an intermediate, and so on.