Here are some advices as to what you can do about saving this.
1. Increase the length of each chapter
Currently the amount of length/chapter you are writing can support 1 ideally, and 3 characters at most on screen.
If you want to actually continue with this many characters and want those characters to feel more than mere dolls then I would suggest increasing the length by about 3-5 times more than your current one.
Your current average chapter word count ranged between 250-390, which is hardly sufficient to convey a suitable amount of information and detail. If you didn't rush things and made things more detailed, then a fic of this kind would easily be above 1000 or even 2000 word per chapter.
2. Reduce the party size
There is virtually no reason to have this many characters on screen, all at the same time. Even rpg games only allow you 3-4 party members at a time.
It would be "best" for the party to split up in some way after Eientei or you can do some kind of switching mechanic where they occasionally replace each other as the party members.
This will increase the quality of future chapters, as the more characters are on screen at the same time, the more text they will leech away from each other, reducing all of their overall quality and possibilities. (The only way to prevent this natural law in story telling is to write even lengthier text)
3. Play around with the current characters before moving on.
Do some Eientei sleep party, slice of life or just do something with the characters overall.
Because all you did so far was advance the story and the characters suffered because of it.
Play around with them by writing non-story focused scenes and let the reader get attached to them.
4. Before adding any additional character, think:
Do I need this character in my story now?
What role will she serve in this story?
Will she be just another bloat to the party?
And decided based on these criteria
5. If you are actually introducing a new character, introduce her properly.
Unless you are intentionally wanting this to be as underwhelming as possible, then I would suggest making new character appearances more impactful, something that catches the reader's eye or makes the situation more interesting.
Or you can choose to not heed my advice at all, I don't particularly care. It is your story after all, not mine.