Unless you do it by working through the Demystified series, of course!

It sounds reasonable to me, a four-year degree at most universities is about half non-major classes.

It's not really a time issue. It's more of a learning issue. I tutored math one-on-one and even of the smartest kids in my cohort found it hard to get calculus; there was a major learning curve for sure. No amount of time will help if the concepts do not click. Feynman-level of physics means you need to understand stuff like variationals, etc which is well above your typical calc course. Shows how truly exceptional this person is.

I don't think variational methods are that big a step beyond integration by parts.

The Lazy Universe by Coopersmith may provide some intuition as well.