UCSD?

My stats are as follows:
UC Capped GPA: 4.0
SAT: 1520/1600 and 21/24 on the essay

ECs:
300+ hours of sports; 10 hours of coaching (however, I’m a junior; thus, I think I’ll be able to complete more by the time I apply)

APs:
I will have taken 11-12 by the end of my senior year; my only AP score so far is from sophomore year (European History; 4)

Is UCSD realistic?

Yes, UCSD is realistic depending upon major. Keep up the good work and keep those grades up.