Look, the first arc is done, the main pairing will get through their issues as they arise no matter what, so the new arc is being written like an original Arai Sumiko work. That's a bit of tone shift maybe but the author draws different angst and lesbian worries from those who aren't Aya or Mitsuki.
I can empathize with folks who wanted more of a victory lap like the occasional bonus content in the manga. That can be nice. Personally I trust those two will be alright and appreciate the story broadening out. If it feels 'too soon' then I think if the manga keeps doing well it'll work out with plenty of fluff over time.
And whether Shu-chan is saphic or not, I kinda like this new plot about some grumpy girl who is a bit uncomfortable with the success and sexyness of those around her. The framing thus far is Shu-chan not at her best as the viewpoint character, so she's probably gay, but it's a bigger stretch to guess pairing when no one else has been properly introduced. It's an interesting choice that Shu-chan isn't likeable thus far, and we have light associations to delinquent characters due to her attitude and bandages. Y'all who complain but couldn't drop reading should be celebrating we got a new androgynous girlfailure who's going to have to grow up.
The kind of plot to expect (not an actual prediction) is this sad designated losing heroine followed her girlcrush to join a club, finds out the girlcrush is gay but taken, finds out she's gay but has no shot, ends up flirting with a girl when she's in slacker or androgynous or butch style and is mistaken as a guy (or vice versa). Because the title still exists. And since she's not Mitsuki she'd screw it up in a different way. That's basically the kind of scope the author is working with - androgynous style and girlfailures pulling girls and having to figure it out. I'm probably way off but I've demonstrated the range of the scope creep.
I guess if I was to wildly speculate the other new girl not even properly introduced as Mitsuki's coworker is the most likely pairing? That's maybe a bit too simplistic Checkov's Lesbian but eh I'm just noting that even at this barebones early stage the author could run with it.
I hope this does well enough to continue as its own thing and not only the celebration and relationship awkwardness of the main couple.