Questions About SAT and ACT

I am planning to take the SAT before the change occurs next year. For people who have taken it already, how many review books do you recommend? Which ones are best? (I currently have the official SAT blue book). Also, what is the change going to be on for next year? Is it the format/content? (I heard the only change is that they are making the essay portion optional). I am also going to take the ACT as well, but SAT is my main focus. Many have recommended me to take both tests so I can see which one I scored better on. Should I double up and study both the SAT and ACT at once or wait until I have taken my first SAT? Like before, how many books do you recommend and which ones (I have “The Real ACT Prep Guide”/red book). My goal is to try to score a 2300+ on the SAT or a 33+ on ACT.

Another quick question about the SAT subject tests: If I score low on one of the tests (low to middle 600s) but score very high on the SAT, will the colleges still look down on me? Thanks so much!