To maximize employment opportunities for bio majors–the more math the better. Applied math is better than pure math. Data science. Computational biology. Biostatistics. Biotechnology Biomedical engineering. . Chemical systems processes (engineering). All good second major, minor ro supplemental course work areas for bio majors.
Unfortunately, paid summer internships for undergrad bio majors are all but non-existent. So don’t count on getting any of those @wishtea.
Admission to grad biology programs isn’t difficult. There are lots of programs that need the “free” labor grad students provide. If you apply widely, someplace will accept you, but getting into a top, well regarded program is extremely competitive.
And unfortunately, even in specialized biology fields, there is a huge glut of PhDs so finding a job, any job with a bio PhD is difficult. The US has been systematically over-producing bio PhDs for >50 years. I wouldn’t count on staying in the US even after finishing a grad degree.