Considering that Kubo's spent 15 whole years with this story and these characters on his mind, it's not surprising that he has at least the general grasp of where the continuation of the story would be headed.
He also said that "it's hard to draw".
I think that it would be nice if we would get special chapters like the "NO BREATHES FROM HELL" every now and then, on a pace that suits Kubo the most.
Also, I would assume (or I would like to think) that he's busy collaborating with the animation team to make the final arc as good as it should be. There's also Burn the Witch Season 2 somewhere down the line.
Anyway, it would be nice if Kubo produced at least one special chapter per year.