PrepScholar was extremely effective for my son, but he’s also a very good test taker in general. Last summer he studied about 25 hours including 1 or 2 practice tests. Before prep he was getting practice SAT scores in the mid-1400s. After PrepScholar he took the August 2019 SAT and scored a 1580. That was a higher score than he made on the practice tests.
We liked that it focused on areas he needed to improve, and did not waste his time on concepts he had already mastered, which were determined by the pretest and progress through the lessons. We never used Khan Academy but it may be very similar. However, you already have PrepScholar. Hopefully you are making progress through the curriculum, passing through the basic sections and then moving into the more advanced topics. Do you feel like you are learning the material? Success on the SAT is partly about knowing the material, but also time management and ability to perform under pressure. PrepScholar has material on these aspects of the test as well.
Summer break is a great time to do your SAT prep. I believe test prep should never interfere with your classes during the school year and your extracurricular interests. We saw students at our high school taking 3 hours classes after school, twice a week, during the Fall semester, taught by one of the big name test prep companies. How they had time for that along with multiple AP classes and other activities, I do not know. My son tells me his classmates thought it was a huge waste of time.
Stick with it and good luck meeting your goals!