As an add-on suggestion to allowing experienced Hearthlings to “train” apprentices and help them level up more quickly, I was thinking that they could write a Journal about their knowledge that the apprentices could read and gain xp to the same level quickly. (not instantly, though, that would be too OP). Every time a Hearthling gains a new level maybe they sit at a writing desk and update their Journal.
It would help if the higher level Hearthling dies or gets kidnapped. Also if they are busy being something else: Herbalist becomes a cleric and a new herbalist is needed. If the experienced Hearthling is alive, maybe they could have conversations with the apprentice so their xp advances quickly.
You could have a special bookshelf to keep all the Journals so that when someone starves to death as a hostage in a goblin camp, the new guy doesn’t have to start from scratch. They just go over to the bookshelf and do a bit of study. Imagine a library of knowledge from many Hearthlings!