Wow, series that are long, with a single unified plot, which actually got translated all the way through, but weren’t picked up by an official publisher (or the fan translation kept going anyway), and which are completely hosted on Mangadex… tall order, there.
I second the recommendation for Helck, above.
Not fantasy, but if you can put up with romantic comedy,
Angel Densetsu (84 chapters) is pretty good. (High school boy who looks like a cross between a vampire and a heroin addict, but is actually a really naive, sincere, innocent-hearted person, transfers into a new school and —
without knowing it — becomes the leader of the school delinquents, while being targeted by both the school principal and the delinquents of other schools. Seriously, it’s a lot better and funnier than you might imagine from the description, and has an unusual art style as well.)
Dorohedoro (167 chapters) is a weird sort-of-fantasy sort-of-sci-fi series.
Hitoribocchi no ○○ Seikatsu (100 chapters) is a cute
non-romantic school comedy, about an unbelievably shy girl who has to make friends with everybody in her elementary school class.
Monster (162 chapters) is of course famous: drama set in post-WWII Europe about a doctor who discovers, and then is framed by, a perfectly skilled assassin raised by the Nazis. Lots of things in it which are as unlikely as an inflatable hammer, but still dramatic and draws you in.
Mangadex went down while I was looking at titles, so I can’t give links or chapter counts, but some others which I remember which are long and complete:
Leviathan — a weird sort-of-fantasy in which the end of the world is
supposed to happen but is not, because one of the key figures who is supposed to cause it refuses. The translation, IIRC, is a little wobbly here and there.
Homunculus — a homeless man undergoes an experimental trepanation from an edgelord doctor, and develops the ability to see people’s psychological traumas when he closes one eye. At first he uses this creepy power to help people, but things gradually get weirder and weirder.
Immortal Rain — this was a licensed sort-of-sci-fi, sort-of-fantasy series which got dropped by the official publisher, and a completed translation was available online somewhere, possibly on Mangadex. In a very-long-post-apocalyptic world (as in “it’s several hundred years since a war which used bombs and biological weaponry to destroy much of humanity, so civilization is starting to recover by now”) there’s a bounty for the capture of “Methuselah”, a supposedly immortal man. The romantic lead — the granddaughter of a famous bounty hunter — finds an immortal man, whose name is actually Rain and who is kind of clueless, but things are actually a lot more complex (and tragic) than they seem.
Dad, the Beard Gorilla, and I — this might or might not be on Mangadex, it got licensed and was taken down but I think it showed up again. A little girl and her (widower) father move in with the father’s (taller and hairier) brother, who she calls “Beard Gorilla”, and as they continue in their lives and meet new people Things Ensue. Notionally comedy, but has some tragic and wistful stuff (about the dead mother).
Tonari no Nanige-san — no real unifying plot, but a very long series of comedy one-shots about a mysterious girl who pops up and gives people in distress exactly the thing they need, exactly when they need it, no matter how unlikely either the thing itself or her ability to know what they need may be.
Also, a couple of long but incomplete-as-yet Manhwa titles which are good (and not apparently hosted on Mangadex, or at least I’ve been reading them elsewhere):
Trash of the Count’s Family (stalled at 92 chapters for now) — this is based on a super-popular light novel, apparently. Guy gets sucked into a fantasy novel as a throwaway drunkard character who is supposed to get beaten up by the hero and die, and uses his knowledge of what’s going to happen to collect the mysterious secret powers which were basically unused in the novel to survive in hopes of being able to retire and slack off after the impending war is over. Currently at 92 chapters, but the translation has had no additions for about 9 months now because it is caught up to the original manhwa and that has not been updated. Mostly serious, but with some comic relief. (I’m told you can read much further in the translated novel than the comic has reached.)
The Greatest Estate Developer — Comedy, very similar initial setup to Trash of the Count’s Family, in that MC is sucked into a fantasy novel as a throwaway drunkard character, but instead he finds that he has game-like abilities which, oddly enough, are related to construction and landscaping rather than fighting. More importantly, though, he has the ability to be an outrageous total a**hole (in a funny way). Between his abilities and his willingness to be obnoxiously rude when people try to stop him, he totally subverts all the events which are supposed to happen in the novel. Currently at around 93 chapters and one comes out at least once a week lately.