I know of no snake that will do that. A snake will, at its best, clear a clogged drain, but the crud in a galvanized pipe is too hard and attached to firmly to clear. If you have other galvanized pipe in your home, it is going to be just as bad, so you are faced with a repiping job in the near future. Flex lines are not supposed to be used where they are concealed, so other than a very temporary fix, that is not a good option. If you want to do just the necessary work, I'd suggest going with new copper pipes from the bathroom to the crawl space. Tie it on to the line there. That way when you repipe, that much is done. Lots of people with older homes are facing this same problem with their old galvanized pipes. It's what we had back then, but now the rust and corrosion is catching up with us. Sad to say, especially with the cost of copper now, but that's what you're facing.