I think the exercises are designed to progress through three difficulty levels: Foundations, Medium, and Advanced. If you’re currently finding it too easy, keep working through the course until the higher levels unlock. While the lessons and videos are generally the same across levels, the practice questions become more challenging as you move from Foundations to Medium and then Advanced. 

You can also take an official Bluebook practice test. Your results can help you identify which skills to focus on in Khan Academy, making your practice more targeted and challenging. Goodluck with your SAT!