I haven't heard most of these, but the first three are so laughably dissimilar to Opeth's acoustic sound (especially Kansas... wtf) that I'm amazed you could just lump Opeth in with them.
edit: And yes, I do realise that Opeth derives much of its sound from various classic rock bands, but I don't think KC, Camel or Kansas are the place to look. To the OP I recommend Nick Drake, since Akerfeldt actually does seem to replicate Drake's vocals, and the music has a similar mood to Damnation.