Nevermind if they'd consider joining, there isn't a sizeable pool of JP indie 4-views period. JP in 2024 anyone past a few hundred viewers jumps or gets scouted into a corpo, whether as themselves or as a fresh debut. There are even more tiny corpo in the JP scene, and these are usually considered more respectable to the JP audience - just how Japanese culture is - even if arguably they're more of a scam than EN small corpos. The Twitch equivalent, NicoNico, has been on life support for a long time and so there's no longer a place for grassroots indie vtubers to grow outside of random virality on YouTube. This has taken away the equivalent 1k-5k range that EN Twitch indies can reach at the top end without big corpo branding to boost them. The handful of extremely successful JP indies are usually converts to vtubing from normal streaming back in 2018/2019/2020, or Niji/Holo retirees.
That's a simplified way of looking at it, but it really is the case that so many JP vtubers both mid-sized and small who might otherwise be worth consideration, are actually associated with random corpos you've never heard of.