Looking online it seems like people generally advise waiting 3-5 years until a sapling is ready for pruning/training, but should that advice be disregarded with a faster growing tree?

In general, waiting can't hurt. I would say cut it back if you feel the branch/trunk growth isn't ideal after the growing season - but excessive pruning/training should really be avoided until it is ready to be put into a pot. I have zero experience with any hickory bonsai but that's how I'd treat one