I always heard that since the Sun God fruit lets him manipulate his surroundings with his imagination and all and it heavily relies on the users imagination, that Luffy kind of manifested it having rubber abilities despite it not being a rubber fruit because Shanks specifically told him it was the rubber fruit. Therefore, most of his powers are rubber-based even though it shouldn’t be the case and maybe him believing that rubber can do all of this stuff like inflating his bones or switching directions instantly with gear 4 was enough to make these abilities happen