I’m certainly not an expert, so take my opinion with a grain of salt.
With a 4.0 GPA and a 31 ACT, you’re significantly above the average admit as of Fall 2017. CLA is not the most selective college at UMN, so that makes you stand out even more so. I would say you should focus on becoming Nation Merit Scholar and focus on maintaining your current work ethic, and when it comes time for the SAT, knock it out of the park. That should give you an excellent chance of admittance to UMN, as well as significant scholarship opportunity. It couldn’t hurt to retake the ACT so long as it won’t interfere with your primary responsibilities and you can prepare enough for it to boost it a decent amount. But if you get busy with taking more APs and Honors courses, then don’t stress about it too much.
Again, let me reiterate, I’m certainly not the most knowledgeable on the matter, but there are plenty of people here that can chime in and probably give you a better idea of what to do.