Get your trunk thinkness how you want and cut it back. The tree will grow new limbs. Best to grow it in the ground for Maximum thickness

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