Licking is not needed for salt. Licking is most often affection, two buns that have a strong bond will groom each other a lot, and licking is important as it reinforces the bond. Licking can also be a signal of appeasement, i.e. The bun signals that it’s intentions are good and friendly.
I think what you are doing sounds good. Treats and head-rubs and ear-rubs are often appreciated by a shy bun. Keep in mind that they have almost 320 degree vision but they don’t see well straight ahead and they can barely see things that are very close. They also fear everything that comes from above, because predators do that.
I have a bun who probably had been neglected. It took him a year to warm up to us, but when he came around it happened very quickly. He now enjoys sitting in a human’s knee and be pet. Which is very rewarding for the human, because having won the heart of a very shy bun really makes you feel special 