Dostoyevsky is a hard read even for native speakers of Russian. I would not characterize him as eloquent, more like raw and rough around the edges. I can definitely imagine how hard it is for a non-native speaker to understand his writing, with his archaic words, words for things that do not exist anymore, references to Russian folk tales, or cultural context of the 19th century Russia, on top of all the things that you mentioned. In a way, Dostoyevsky is somewhat watered down Russian precursor of James Joyce.
But Dostoyevsky is brilliant, his impenetrable style notwithstanding - in Crime and Punishment he paints such vivid scenes that I can still in my mind’s eye see Raskolnikov’s crappy apartment, the widow slumping to the floor, axe in her head, the markets of St Petersburg, the reeking drunks by the stables... it builds into something glittering, always, even when painted only in dun and ochre.
Joyce is an apt comparison - he’s similarly a painter with words - I feel like I’ve visited 19th century Dublin having read his works, although he does make Dostoyevsky look sedate and restrained.
