UCR is a very realistic goal, and this early-assurance program is unique in the UC system - about 24 UCR upperclassment are admitted each year. Thomas Haider Early Assurance Program | School of Medicine Student Affairs
Stanford is an extremely long shot for any unhooked applicant, especially those from California. You’re an excellent student, but virtually everyone who applies to Stanford is an excellent student, and fewer then 5% of them are accepted. (And when you look just as those who aren’t recruited athletes or legacies, the odds are even lower.) Certainly, give it a shot, but don’t invest all of your hopes in Stanford - it is a super-reach, and you need to find reach, match, and safety schools that you would be happy to attend. Riverside will likely fall in the match category, but it’s too early to say for sure.
Are you sufficiently low income to qualify for Questbridge? If so, apply next year for their College Prep Scholars program - the application opens in the winter and is due in March of your junior year. QuestBridge | College Prep Scholars Program If you subsequently become a finalist in their College Match program, you can rank up to 12 schools among their list of partner schools (including Stanford, fwiw) and potentially qualify for a full ride at one of them. This is the best aid package available, at some of the best schools in the nation. Even if you don’t match, finalist status is an advantage in the regular application cycle.
Run the Net Price Calculator for Stanford - this will show you what your best case scenario looks like for need-based financial aid. Estimate Cost : Stanford University Also run the NPC for UCR. You need to determine which categories of private colleges and universities may be more affordable for you than the California public U’s. (I assume you’re a California resident, yes?) If your costs at Stanford would indeed be lower than at Riverside, then run the NPC’s for some slightly less generous (but still excellent) full-need-met privates, like Pitzer and Occidental. The goal is to filter first for what will be financially viable for you, and to figure out what your financial safeties will be - schools that you know you can get into and know you can afford.
Also make sure you take the PSAT in the fall. Put the Khan Academy test prep app on your phone now, and get in the habit of using it for 10-15 minutes a day - nothing strenuous, just consistent. A high score on this test can boost your prospects. Standardized tests have become somewhat less important in pandemic times, but it’s still worth honing your test-taking skills. The MCAT is a significant factor in med school admissions, and many of the same skills and strategies apply, so it’s worth starting early at becoming a good test-taker. (Again, not something to stress about or devote crazy amounts of time to, just small amounts of time on a consistent basis.)
You’re going great so far - good luck!