So I’ll double major in biology and something else I’m actually interested in, say my intended career is medicine (even though it’s not), and maintain my low interest rate for four years (or even longer)?

One would hope that calendar time paying post-graduation >> time paying pre-graduation. ;)

They're 20 year loans.

I'd be hard-pressed to think of a single person that wouldn't tell an unverifiable lie to get a lower interest rate on 20% of their loan lifetime.

Doubly so for loans with compound interest (as some private student loans are).

So just roll the difference for your entire study period onto your post-graduation rate?