I think the narrative works just fine as is, without changing the intro sequence. Introducing unrelated characters in a fanfic is not uncommon and does not make the story boring at all. If you wish to cater to people who lose interest very quickly, I would suggest expanding upon the protagonists a bit more before they meet A-chan and delay all the lengthy exposition that is contained in her stories. Making the trio meet other characters related to hololive would be cool, them making friends, butting heads, basically having a bit more action around that part.
At the same time you expand upon the protagonists and show what they think about certain livers, you also give readers fan service. Here you can also explain a bit more about the current situation around the kids, how they live, what they eat, where are their parents and such. What exactly do they do day to day, where they hang out, what are their personalities, strong sides and weaknesses. You make readers spend time with them to grow fond and empathize.
Then you hit them with a bit of exposition. In your place I would cut A-chan's stories into parts, and weave them in from time to time between the misadventures of the kids. That way you show that time is actually passing in real life, between sessions of book reading when kids go to meet her. It could also help if the story from the book reveals some information that comes up in the current time frame, maybe they see something related to advent in somebody's house right before or after the A-chan meeting, or find a broken clock in the garbage dump when they get thrown out at the start, or something like that.
Honestly, even though them meeting a holo talent early on is very cool and exciting, it is not something you can replicate again without losing interest later on in the story. I can understand this narrative device working to hook readers, like a pilot episode in a tv series, but on your place I would be spending just a tiny little bit more time to build up expectations. Make them hang out maybe with other fans like chattini or seeing kronies from afar. Make them spend time and build relationships with lesser known characters in holo lore. Even holostars are probably a good fit for this just like Magni, maybe make him more important to the kids early on, maybe one of them can have something in the past that is related to him directly.
Delaying the meeting between the main trio and any major hololive personality would fit better for a story that actually takes itself seriously. Make it clear to the readers that cool and awesome talents that everyone knows are a BIG deal and no common people can just go out and hang out with them out of the blue. Make the narrative more grounded, and slowly allow the kids to earn themselves a position where they can meet a real talent.
But that is a preference choice for the writer. Making the story more light-hearted with kids just casually hanging around all the holos early can work just as well. Sorry for the long ass reply, I just like your story and want you to realize the vision the best way you can. This advice is only my opinion.