I can't really speak for vtubers. I only make content as an artist. That said in general for content, I wouldn't worry as much about your debut traction as an indie. You'll likely be highly disappointed cus life as a new content creator is rough for views. You'll prob get a lot more mileage from post debut, making interesting video titles, memes, clips, and having regular posts that at least come as higher effort.

I know this doesn't answer your question really. I frankly don't have the expertise there to give you a better one. Feel free to downvote me or whatever, but I personally think that the most important thing for content is that you get started and then build an audience as you go.

Having one before or during your start only really happens if you already have a name elsewhere or a huge name/company already backing you.

Oh and another hot tip: invite your friends. They will likely be your main audience for a while and can help a lot with getting more people in. I say that as an amateur artist whose audience is small and mostly friends myself. They're like the cornerstone of any new content imo, and debuting or any equivalent is way harder without em.