If each character was under a great arcana, I'd say that Azriel is the Hanged for failling to his task, Berg the Tower because where ever he goes his shackle or the taint of the Mara will fuck shit up, and Black Gale the Fool discovering the World.
The Hanged, The Damned and The Fool
I bet that Azriel wants to do good deeds so he can ascend to Heaven again, and that's why he hangs out with Berg. He kinda survey the travel of Berg's soul because he met him at the end of his quest. I think he considered this meeting as a sign of the Fountain for his "redemption quest". And you can feel the saltiness in the roleplay "Why do I have to take care of this dumb orc" kinda vibe !
It's an hypothesis of course, but sessions like these give great insight on the character.
So I don't think that characters are uninteresting, we just need some times to develop the depth of their belief. Of course if those question aren't adressed it's gonna be tricky.
And it's hard in a dungeon grinding campaign, let's not forget that this campaign is heavy mechanism oriented and the guests are the narrative hooks to explore new lore/dungeons rather than indepth central plot exploration !
It would be possible with a guest that would stay more than 4 sessions tho