Where did you get your UCSB rankings from? According to US News ranking of National Universities UCSB 37 / UCD 46. College Niche ranking of top Public Universities has UCSB 18 / UCD 26. IMO, after UCB & UCLA, the two UCs are in the second tier and are both great schools.
You say you are worried about giving away a “golden opportunity” but not going to UCSB but it doesn’t even have your exact major like UCD. Also, both UCs have 1,000’s of asians (and I’m guessing your STEM science major will have a disproportionate number of asians as well). You will be fine at both campuses. Lastly, a flight from Sac (20 mins from Davis) to Burbank is easy and cheap on SW if booked in advance if you are worried about being so far home. With traffic SB, could be 2.5 - 3 hours from LA.
I loved my time at UCD and my little sister went to UCSB and also had a great time. At the end of the day, you will have a wonderful college experience at either college Good luck!