I just started watching :Re and I feel like if the quinx squad adds nothing to the story. If they just weren't in the series and everything they did was just done by already existing ccg characters I think the plot wouldn't lose anything. Shirazu is okay I guess, he adds a little to the story, and maybe tooru but the entire group just feels like they were forced in to add "character/personality" to the show rather than actual quality and are all just walking stereotypical anime archetypes:
-Shirazu(not annoying): archetype is the dopey/kind of dumb guy who is hyper and energetic and makes rash decisions but is smart/analytical sometimes and is a genuinely good guy
-Tooru(not annoying): archetype is the very nervous newbie who is still weak but has a lot of potential and is very shy but nice
-Urie(very annoying): archetype of "guy who acts like a dick because he has low self esteem/is insecure and wants to prove himself and is angry at the world because his dad died" what really annoys me about him is that: (a) his only thoughts are talking shit about everyone else and trying to be better than them (b) he blames every other fucking character for his father's death when only one character was actually related to it in any way whatsoever (c) Kuroiwa was ordered to retreat but urie still acts like he personally killed his father (d) is constantly bitching because his daddy is dead when literally every other major character has one dead parent, most have two(hell, hinami's mother was murdered right in front of her and i'm pretty sure touka's father was too), and have had completely shit lives aside from that as well, but they don't blame everyone else in the fucking world and aren't constantly whining. Also this trope is overdone, we've seen it in nearly every action anime
-Saiko(most annoying): archetype of "weird quirky neet girl who's always saying le random funny things rawr xD" she is the most annoying because she is literally useless and her archetype is pretty overdone in anime, but that's not the worst part. the worst part is that she is just forced humor and adds the least to the plot, and she isn't even funny, her "humor" is the most forced shit ever that only a middle school emo kid would like, in my opinion she's just fanservice for pedos and hentai addicts.
I mean yeah, some people hated them when the manga started, but they became an integral part of the story later on. :Re is a fresh start from a new perspective, a new Kaneki and an set of new characters. Each of the Quinx shared some parts of Kaneki's personality/struggles but the way they deal with it varies from how Kaneki did so. They don't add a lot to Kaneki's story but their growth and development was fascinating to read.
Their concept worked brilliantly in the manga, some of :re's earlier arcs were the most immersive parts of this series. But the anime adaptation is so poor that the best arcs of this series are considered as trash and Quinx who progressively became more interesting are completely ignored by anime onlies. Since you haven't caught up yet, I'm leaving some earlier impressions I got from their anime and manga counterparts:
-Urie in the earlier stage of manga was a jerk but badass, by far he had the most interesting character design and most memorable action panels out of the four. He was often drawn without a mouth (implying he is biting back his thoughts) or had his entire face shaded in black except his cat-like eyes and grinding teeth (implying his inner devil is being expressed). Almost each of his dialogue box had a part which was bound in parenthesis to reveal inner thoughts which made him transparent to the readers. This feature is a pivotal part in his character arc which you'll understand later if you decide to read the manga.
Urie in :re anime has the most bland character design that can possibly be drawn for the 'cool' archtype (I didn't know it was even possible to make a character with such distinct features look so two-dimensional). His mental breakdown was cringey and he looked way older than he should. If I was anime-only he'd probably be my least favorite which is kinda funny because he's my favorite character in Tokyo ghoul and I liked him from his introduction in the first chapter.
- Everything I can say about Mutsuki has to be masked as a spoiler because the anime will not bother to let you anything regarding this character.
One of the first things revealed about Mutsuki in the manga was that Mutsuki is biologically female with lots of scars in the body. Male gaze made Mutsuki feel uncomfortable so Mutsuki wanted to live as a man before undergoing Quinx surgery. Mutsuki was easy to like but some hints were dropped that Mutsuki is very secretive and hiding a lot about own self.
The core part of Mutsuki's character arc is non-existent in the anime, and without reading the manga it's impossible to understand Mutsuki's drive/motivation/deeds. It's a shame indeed.
-Saiko in the anime looks more..pretty? Personally, I always liked her and found her little jokes funny. She's not an underage btw, and as far as I remember the manga didn't really sexualize her. The anime doesn't tell her background and why she's in CCG in spite of having no intention of fighting. It makes her look as if she's just there to be there. I can't confidently say your opinion on her would differ a lot if you read the source material because she's like a hit or miss. Most people tend to like her though.
-Shirazu was done pretty well in anime adaptation. Again, many of his background information are missing but it doesn't take away a lot.
As for myself, I randomly read the first chapter of :re and found its premise, characters and art-style more interesting/engaging than the original Tokyo ghoul's, and I read the original manga to catch up on :re (I really enjoyed the final arc of the original Tokyo ghoul manga though, which was not animated.) Well I'm one of the minority but most manga readers would agree that early arcs of :re had some of the more interesting parts of the entire franchise but it's unthinkable to anime-onlies.
I can say this series could be AOTS material if it was adapted faithfully. We're unlucky, I guess.