Everywhere I read about his origins it says he originally served the Imagawa but left, and then skips directly to him being Nobunaga's sandal bearer. What happened in between? Was he already something like a high-ranking ashigaru? I imagine it requires earning trust and some merit to get to become sandal bearer to a daimyo. Additionally, what would his job entail as a sandal bearer? Was it literally just keeping the daimyo's sandals at his disposal? Would he also act as a bodyguard, or run errands and other similar tasks?
Yes, a sandal bearer was literally just keeping the lord's sandals at his disposal, a member of the lord's household servant staff.
Sources differ on how he got the job after leaving the Matsushita and when he joined Oda Nobunaga:
I believe there are other stories as well. All of these come from narrative sources in the early Edo period (at the earliest), so we don't know which, if any of them, are correct.