Should I retake the SAT? When should I take Subject Tests? (math major, if that makes a difference)

Hey,

I’m a prospective Math/CS applicant to T20(?) schools. I last time I took the SAT was August (the beginning of Junior Year) and I got a 1530 (800 Math 730 Reading). Should I retake the SAT sometime in the summer (if that’s still happening) or in the fall? I didn’t do any studying the time that I took it and was going to take it again in June, but now that’s cancelled. I’m sort of having second doubts about If I even have time to take it, or if I could guarantee a higher score. For reference, I didn’t get any math or writing wrong, but I got 7 reading wrong on the August SAT. When I took some reading practice tests, my scores were really volatile. One day I would get an 800 if I enjoyed the passages, and another day, I would get a 1510. I don’t have any issues with the Grammar or Math sections.

I also have to take one other Subject Test. I already took Math II Subject Test and got an 800 and I’m planning to take Bio, Physics, and US History on the next available date. Is it still possible to have take an SAT Subject Test AND and SAT retake before the next admission cycle starts?