Okay, I have looked through the forums and similar threads but the problem is... I think I have no life. I've literally read everything people seem to be recommending that is slightly popular. 😐

Are there any hidden gems out there I haven't read yet?
What I'm looking for is something long (at least six volumes). I like cute girls and romance but I don't really care if it's fun to read (eg. Haikyuu!!).
I do not like moe art styles and I'm not into yaoi.

Sorry if this is too broad...

I just threw in everything from my completed list that was long enough and not on any of your lists. Hope i got at least one right 🙂

Basara
Akuma de Sourou
Bokura ga Ita
Chocolat (KUBONOUCHI Eisaku)
Eyeshield 21
Ghost Hunt
Good Morning Call
InuYasha
Kemonotachi no Yoru
Kanata Kara
Pinocchio
Tenshi Nanka ja Nai

Aku no Hana
Black Cat
Bloody Monday
BM: Nectar
Bremen
Eden no Ori
Peach Girl

I FEEL YOU!
I look for suggestion in the threads but it's either I've read them already or it's uninteresting to read.

Well try Uchuu Kyoudai, it's not complete but long enough.
Very enjoyable story and inspirational.

Koibumi Biyori (3 vol. One shots)
I know it's not entirely what you're looking for, the art is not moe but not that great either. The story though is very charming.

Anything by Ikuemi Ryou is guaranteed.

We have a lot in common on your lists. Maybe check mine out and pick the unchecked one. Lol.
(Sorry too lazy to list down)

Thank you all!
I've read almost all of these, but the few I haven't I'll read now lol

Hinamatsuri
Helck 5 Volumes but I had to recommend this.
Kingdom
Mob Psycho 100 OPM author
Shuto Heru
Spirit Circle author of Hoshi no Samidare
Feng Shen Ji

Bit darker, completed stuff:
Sekai Oni
Kimi no Knife
Kokou no Hito

My top recommendations that are complete and not already on your lists would be:
Kyuukyoku Venus
Goong
Eien Kamoshirenai
Tenshi ja Nai!!

But since this is a pretty broad request and you seem to have read a lot, I'm just going to list a whole bunch of other good options...
Complete:
Alice 19th
Full Moon o Sagashite
Fushigi Yuugi and Fushigi Yuugi - Genbu Kaiden
Girl Queen
Juuoku Shoujo
Kamikaze Kaitou Jeanne
Kemonotachi no Yoru
Land of Silver Rain
Legend of Nereid
Shinshi Doumei †
The Queen's Knight

Still being scanlated and/or written (but past the 6 volume mark):
Koibana! - Koiseyo Hanabi
Habaegui Sinbu
Never Give Up!
Ookami Heika no Hanayome
Sakura Hime Kaden
Scarlet Palace
Shibariya Komachi
Soredemo Sekai wa Utsukushii
Umi no Kishidan

I do not see Nijiiro Days (MIZUNO Minami) in those lists... Is that a bug? 😛
It's ongoing though, but I didn't see any completed requirements.

in need of romance?
uncommon, sad, sweet

I recently reread Kekkaishi and that was long...

"I'll shut your mouth~~~~~ with mine~~~"

二息歩行

Here are some of my favorites that fit the length requirement

B Gata H Kei
Beck
Love Roma
Onidere
Aa Megami-sama way too long but I had fun
Mahoraba
Tasogare Otome x Amnesia

these are some that i have finish reading.

Attaque
Akumetsu
Bakuman
Baljack
Bambino
Dendrobates
Hareluya Il Boy
Mx0

Hope any of this is of your liking.

you can look in my complete list, most of them are long and i just put a few of them before.

Not sure what you mean by fun read since I'm not sure if you mean "light-hearted" or just good in general even though in some, or a lot of places, places it's sad/etc.. So here are a few suggestions:

Light-hearted:

My Girl (SAHARA Mizu) (Complete, Slice of life/raising a child that's only 5 volumes, though close enough I think 😀 )
Eyeshield 21 (Complete, American Football manga that is a lot of fun)
Cardcaptor Sakura (Complete, A very good magical girl manga)
Chrno Crusade (Complete, Action Adventure Historical with Demons)
Arakawa Under the Bridge (Complete, living under a bridge)
Muhyo to Rouji no Mahouritsu Soudan Jimusho (Complete, fun Supernatural read)
Sugar Sugar Rune (Complete, witches and fond memories, but I did read this a while ago)

Ookiku Furikabutte (Ongoing, baseball manga that is a lot of fun)
3-gatsu no Lion (Ongoing, shogi player finding a place where he belongs)
Giant Killing (Ongoing, football/soccer manga that's about the coach/team, not the players)

Good in General

Hellsing (Complete, there are vampires and Nazis)
Oishii Kankei (Complete, cooking manga that I felt was similar to Ashita no Ou-sama in some ways)
Kaijuu no Kodomo (Complete, very interesting manga about the sea)
Oooku (Ongoing, Historical w/ matriarchy basically)

Piano no Mori (Ongoing, about becoming a world renown pianist)
Capeta (Ongoing scanlation but complete, about kart to f1 racing)
Shoukoku no Altair (Ongoing, Historical with Turkish influence)
Sidooh (Ongoing scanlation but complete, about samurai)

And if you can't find anything more to read in the Asian comic world, try Western webcomics like these (they're fairly long too):

Unsounded (~819 pages)
Trying Human (~751 pages)
Widdershins (~599 pages)
Never Satisfied (~105 pages)

Must Read Webcomic:
Unsounded

Lists:
Reading
Complete
Western Webcomics

Thank you all! I have read most of these, but seeing them again made me wanna reread them
And lol yeah, I forgot to add niijiro days to my faves, I love that manga

You should try Ciel:The last autumn story[/m][https://www.mangaupdates.com/series.html?id=2345]
It's got romance, is 22 volumes long, I think...and it's seriously good, give it a try. The fema,e lead is perfect, I thought the middle part was a bit confusing but if you keep reading on the manga just gets better, especially when Yveine grows up, the story does have multiple protagonists but the main one is of course Yveine, the others are kind of side characters with their own stories

This isn't romance but Gintama. I love to ship Kagura and Okita, so to me it's alright
[https://www.mangaupdates.com/series.html?id=228] I think it's a lot better if you the anime though, it's up to 400/500 episodes, I think.

You should try Meteor Methuselah or Immortal Rain
[https://www.mangaupdates.com/series.html?id=5348]
It's romance too, it's pretty long. I think it's a really beautiful romance with a lot of action. Try it.The female lead is so cute and strong so that's a plus.

Oh there's also 7 seeds,
[https://www.mangaupdates.com/series.html?id=548]
which I think is awesome (' 😀 ') That's a romance too! And don't drop it because of the art, it's amazing 🙂 Give it a try, there are multiple protagonists so you get to choose a side. The plot is very well developed and it's just so cute...

😀 😀

Your mum

phew, good. I was sure I saw you around in some threads so I was a bit confused^^
If you are still looking for stuff when you are finished with those recommendations, do tell, and I'll try to dig up some more, that one just immediately came to my mind.
have fun

in need of romance?
uncommon, sad, sweet