Just my opinion...
Astronauts come in a variety of "flavors", but you can shoot for the Mission Specialist route or the test pilot route. The Air Force will be good for both of those (but I'm a former AF Captain, I might be biased ha ha). Pilots that are scientists are always good.
Mission Specialists are scientists. Some advice I can think of would be to study the astronauts that have flown in the past and ones that are in the program now. You can probably find that info on the NASA page somewhere. You should study something you love, and if you look through the list you might find something you like more than Physics and Astronomy (but I just can't imagine anyone liking anything more than those two subjects
).
Also, it is HIGHLY competative, so you need to get GREAT (no FABULOUS) grades, have LOTS (and lots and lots) of leadership roles in college (which any military program would have), and you need to be in GREAT shape. Also, you need to stay out of trouble. (I can't stress that one enough)
If you start off knowing what your goal is, you can be bold and check all the boxes. Your Dad's advice is very good, I would bet that most astronauts have physics degrees out of the AF and are pilots (but your research can tell you that).
GOOD LUCK!!!!!!
Tina