Have you seen Haibane Renmei? I just finished it the other night and it seems to fit in with everything you listed about Mushishi. I absolutely adored it, and it hit exactly the same strings in my heart that Mushishi did. Just know that it's a lot more emotionally heavy than Mushishi. You never have to linger on the depression that Ginko comes into contact with, but you have to go through the depression in Haibane Renmei right alongside the characters. It also doesn't explain every mystery like Mushishi does. It's very Ico/Shadow of the Colossus/The Last Guardian.