The collaborative efforts with Holostars is kind of strange, and it's honestly not really Cover's fault. YAGOO's twitter actually promotes the Holostars milestones more than it does for any of the female talent. Holostars actually suffered a lot because for a while they didn't have a proper branch manager, and now that they have one he's become kind of a character in his own right. Fubuki and Matsuri give absolutely zero fucks about what people think about them collaborating with the Holostars, and I forget who else it was (maybe Subaru?) but someone from hololive also basically told her more spergy viewers to fuck off and she can be friends with whomever she wants to be friends with. It's also more or less an issue of scheduling; a lot of the girls are supremely busy with many other things than just streaming (like that music video Subaru, Aqua and Pekora made for instant noodles) and have a lot of prep work they either have to do on the side or choose to do in order to help further the brand; Fubuki and Coco in particular do a lot of BTS stuff and Coco spearheaded the EN branch opening and was part of the audition process in terms of screening. Even that aside, any time one of the girls works with a Holostar the video gets dislike bombarded because how dare their waifu have a male friend, and some of the talent may be actively avoiding something like that. It's not really a problem with the overseas fans since most people realize the girls are just actresses and are free to have outside lives, but Japan really is hung up on the "seiso idol" bullshit that hololive is gradually eroding. I can't imagine what will happen if the Japanese find out that their cute shark loli was a fucking degenerate who would regularly get shitfaced and make shit raps based around furry copypasta.
The only consistent person I see who does collaborative efforts is actually Roberu, and that's just because the guy basically lives at his computer. If you take a look at his streaming schedule, he goes live several times a day; once in the early morning for a free talk with his audience and then later in the early afternoon to play whatever he has planned for the day. He never really takes a break, and because of his open schedule he can work with a lot of people outside of holopro, like the VOMS girls. Unlike a certain other shut-in who lives in what is considered a fucking streaming closet though, Roberu actually has a lot of charisma and love for his job.
Nijisanji tends to be absolutely wild with whatever characters they come up with and they don't really follow the idol playbook that Cover does, which in honesty may have just been an afterthought. Cover was a tech startup that saw a market in Vtubers because the Chinese were throwing money at people like Fubuki, when really they just wanted to sell facetracking software and used their idols as mascots for what it could do.. Ichikara was a Vtuber agency from the very beginning, although according to their flagship girl Tsukino Mito, was actually terrible to work for early on. I have no idea how she stuck around that long but she definitely suggested things so that the company stopped treating its talent and its BTS staff like absolute dogshit. They also don't really use the "idol" tag at all, instead just calling them "livers." They also have a lot more freedom to talk about their personal lives; Gwelu Os Gar is a dad and his viewers congratulated him on both his marriage and the birth of his kid when he talked about it. Imagine how the hololive's Japanese fanbase would react if by some chance one of the girls was an expecting mother.