Hmmm, fair fair.
I guess if any girls out of the bully trio are going to try their luck getting with Ueno then it would be Chisaki, I'm still not convinced, I honestly think the author will play out the rest of this series relatively straight.
But if Chisaki and Ueno do get together, I think it is more likely it will happen IF things don't work out between Ueno and Moka, Ueno isn't willing to cheat on her and even if his body still submits to the girls he won't let things get that far, he rejected Tsubomi and only orgasmed for Kanoko because she literally forced the kiss and he got caught off guard.
Let's assume Chisaki is willing to steal Ueno from Moka, two massive problems now are that Ueno wants to move forward and Moka now knows about everything the trio has done, there's a good possibility she's going to be protective of Ueno so if Chisaki wants to do anything she would need to catch Ueno alone and even then, Ueno has shown he can resist her even if he later is shown being pent up.
No, I don't think the girls are working together, none of them are even fully aware of what the other girls have done to Ueno and frankly when that revelation becomes apparent that might blow up in all their faces and stop any stealing or further sexual advances on Ueno, Chiaski in particular probably would have some strong feelings if she found out Kanako tried to sleep with Ueno and I think if Tsubomi becomes aware of what the other girls have done then I do believe she would end up siding with Moka in a sense that she's not going to let Ueno be treated that way anymore, I do think Tsubomi has been the most honest about wanting to move on from what she's done before.
Honestly, I'm not even convinced the trio's friendship will survive when everything is out in the open, Kanoko herself has commented such things before when Ueno made it clear he wants to date Moka, if their friendship does survive then I can see there being a rift for awhile.
As for harem, I don't think that's on the cards for the ending, pretty sure the harem tag is only a thing for this series is because Ueno has a harem of girls bullying him that aren't aware of the other, as far as the ending goes, I only see two endings at the moment:
1. Ueno stays with Moka after they work through all their problems.
2. Ueno breaks up with Moka but they remain friends and continue striving to improve themselves ala "Looking to the future" type ending.
Just a couple of closing thoughts for this post. For the sake of clarity. I respect your opinions
Firstly, I want to stress about the "girls are working together" statement. I didn't convey this well.
There is a certain particularity about friendships that makes one more... lets say, tolerant of another's inhibitions.
So in this case, consider the question: If Chisaki were to make a move on Ueno, what would Tsubomi and Kanoko do?
As in many of the flashbacks and backstories, Chisaki is the defacto ringleader, this doesn't necessarily mean Kanoko and Tsubomi
want to bully him, but they take part!
This is an example of 'working together' that I wanted to originally convey, they band together when the occasion simply fits. But does not necessarily mean they desire to do so, they simply conform to the circumstance, and become enablers, whether willing or not
Kanoko has, on several occasions, not only expressed she loves Chisaki, but also, that she hates Ueno. And she literally tried to tie him up and lock him in her house.
This can, I suppose, be a front. But that essentially is two-pronged.
Most people, in particular friends circles work this way.
Its why most people are unaware when they are enablers. I do not believe Tsubomi or Kanoko will necessarily interfere or even stop Chisaki if she did try to do anything. But I think they would be more likely to enable Chisaki, or simply stand aside should she have her way, as opposed to the inverse
Secondly, the harem statement.
I'll say it just for the sake of saying, no! I do not believe Ueno will bag them all like most harem slop that leads to fatigue
But this is moreso, a status quo if you will
I believe the author will keep a character's certain element for the sake of the story's drive, and harem being one of them, essentially means Ueno has multiple potential love interests, regardless of their current state. And as such, will frame the story to maintain this.
To some degree, people could look at the current state of Tsubomi & Ueno's recent development, figure out its a resolution of sorts and call it a day! Or the author can keep her function continuous in the story for more possible developments.
But if every character reaches the same resolution and they get picked off one by one, the story loses a core principle of its premise, and that isn't always a good thing. This of course depends on where the course of the story actually ends
and we do not know this.
Anyways, enough rambling on the matter. I respect your opinions

Cant wait to see how the story develops from here on