The greatest German prose writer of the early 19th century and maybe ever has not yet been mentioned: Heinrich von Kleist. (He also wrote one of the few Geman comedies that actually work: Der zerbrochene Krug. And he ended his short live at around 30 in a double suicide with his fiancée.)
Kleist did not write any novels but a dozen or so of shortish novellas (ranging from the length of a few pages to the extension of a short novel). I don't know how well his style survives translation. Despite very long sentence, his style is very concise. His paragraph-long sentences are often breathless eruptions, highly charged. Almost the opposite from the detached armchair irony of Mann about a century or more later. Correspondingly, the subjects are emotional, sometimes gruesome. The most famous ones by Kleist are probably:
"Michael Kohlhaas" about a peasant and horse trader becoming an outlaw because he was wronged by a nobleman in Lutheran Germany.
"Die Marquise von O." (The marquess of O.) which nowadays must come with all kinds of trigger warnings about a young woman who becomes pregnant under unusual circumstances.
"Die Verlobung in San Domingo" (The betrothal in San Domingo) which takes place during the Slave revolt in Haiti (I think).
Others that are quite highly regarded are
E.T.A. Hoffmann (who has been mentioned already) with Kater Murr, Cardillac (aka Madame de Scudery) and The Sandman as maybe the most important ones
Theodor Fontane who is sometimes compared to Flaubert (but maybe mainly because a lot of his novels are about adulterous women)
Wilhelm Raabe (of whom I have only read one book that did not leave much of an impression but he is considered a classic)
And while I do not share the verdict that all or most German philosophers are unintelligible, the only major one who really was a brilliant writer, aphorist and stylist (many think that he was better in this regard than as a philosopher...) is Nietzsche. Schopenhauer is a decent writer, Hegel rather horrible (and often impenetrable), Kant usually dry as dust but actually better than his fearsome reputation.