Welcome to the UCI family of anteaters and bio majors!
🙂
As an incoming fourth year who was once in your shoes, I just thought I'd give you some advice. I'll preface it with this--Humanities Core's prompts and lecture assignments/exams were very similar to the type of writing I had done in high schoool, so I came in well-prepared. I would suggest taking HumCore, since it takes care of two requirements at the same time--humanities and writing.
I am not very good at writing, and received mostly B+s/A's between the lecture and writing component of HumCore, but I think it was worth it and would not want to take a course that was solely focused on writing.
That being said, the decision to take Math 2B and Humanities Core or Writing will fall on you. You'll need to decide based on your own high school preparation and how well you feel you have already mastered the subjects. As someone who took a lot of AP courses in high school, my first and most of my 2nd year was a lot of review of things I had already learned in high school. Honors general chemistry, bio 93, 94, 97, and part of 98/99 was review of what we had covered in high school. In AP English, we read a lot of the same authors that we had read in Humanities Core, so it wasn't too unfamiliar, but I won't say it wasn't a lot of work. If you have had sufficient exposure to the material or feel that you can spend time to "study the material" (and are prepared to devote a lot of your time to studying if that's the case) then you should be okay!
One last thing I wanted ot mention about Math 2b. I got a 5 on the AP Calc exam, so I don't know what 2B is like, but I did take Math 2D, and the course was ajoke, because my professor was a visiting prof who gave us practice tests that were identical to the actual exam. If you're lucky to have a good professor that can teach well AND makes easy tests, math will be a cake-walk. Also, a strong background in calculus will definitely help you. However, if you don't feel you've received adequate prep, be prepared to face some mild difficulty in math. I would honestly push Math towards later years, and take some summer school classes in order to "spread out your workload". Granted, 16 units/quarter is not too heavy, but not too light. You can probably do it if you have sufficient time to study.
Sorry for the long-ass post...good luck!
Do ask if you have any questions!