“I’m also not super worried about the interview.”
Then you should be. They are about finding the smartest people, not about whether you’ve had “exposure to microbio techniques and applications”. The interviewers don’t want to know what you know, they want to know how you think. It is such an intense experience that many candidates can recall their interview years or even decades later.
The reason a 4 on an AP is a red flag is that these exams are easy compared to the rigor of UK A-levels (although they may be a bit more forgiving for an AP taken as a sophomore). And nowadays most candidates get a lot of interview prep too, which puts US students at a big disadvantage. Will your teacher reference say you were their best student in the last 5-10 years?
To stand out in sciences, participation in Math/Bio/Chem/Phys Olympiads is highly valued (probably more so than your internships since that’s not really a thing high school kids would do in the UK). Try doing one or more of those next year. Remember that maybe half a dozen kids per year from CA will go to Oxford across all subjects combined.