I'm coming up on my senior year, and I want to study Computer Science at UMASS Amherst. I have a good GPA (around a 4.3 weighted), and a 30 ACT score, so I think I have a solid chance of acceptance. What should I prioritize at this point, maintaining a rigorous course load that will show up on my transcript or self-studying for AP tests that will give me a head start in college? Would it be a bad idea to scale back my actual course load senior year to leave room for self-studying?
You shouldn't reduce the difficulty of your course load senior year. Colleges will know about this, and it may look like you're using senior year as an opportunity to slack off. Continue taking courses at the same or higher level as the ones you've taken in the past. If there is an additional AP test that you want to take for college credit, you can consider doing that as well, but don't try to take a million AP tests at the end of your senior year for credit.
Even if you get a couple more college credits or are excused from some introductory classes, taking an AP test is not a direct substitute for a college course. If you are placed into a higher level course based on your AP score, you may find that you're not fully prepared to take on the material. That's not to say that AP tests are useless, but if you plan to reduce either the difficulty of your course load or the number of AP tests you take for credit, you should sacrifice the AP tests first.