For the more acoustic side of things, I'd recommend you check out iamthemorning, The Moulettes, Agalloch (also has some metal aspects), and maybe some of the older 70s prog bands if you aren't too familiar with them (like Camel, Yes, and Floyd). Anathema, Frost*, and Pineapple Thief are also some good bets.
For artists similar to what you listed: Ne Obliviscaris (most similar to Opeth), Kingcrow, Russian Circles (mostly post-metal/post-rock and they rarely have vocals), Agalloch, Dir en Grey (warning: those vocals are wild), Anciients, dredg, Leprous, Primordial, Soen, Wilderun, and Arkona (mostly folk metal, with an emphasis on the folk. They have some really upbeat stuff too). Dvne, Tool, and Mastodon might also be worth checking out if you haven't already.
And for something in the same vein, but totally different: check out Submotion Orchestra (Jazz meets R&B meets Electronic music), Miles Davis' Kind of Blue and Birth of the Cool (jazz, good atmosphere), impressionist classical composers like Debussy and Ravel and late romantics like Mahler, Liszt, and Wagner, and stuff like Lorde and The Weeknd (which are pop/R&B, but have the same atmosphere you're looking for).