I think both options are doable and would really just base it on what you, personally, feel best with.
Like, if you're really hyped about starting another idea while you're building up your stockpile, then just go for it regardless of what anybody else says! If you don't really want to and are completely immersed in your first project but think it might be good for attracting an audience (but don't necessarily do this as your job), then maybe don't start something new just yet and instead focus on the first one.
If you're really unsure, maybe ask yourself some questions: Like, how much time do you have per day to sit down and write? How long do you need per chapter on average? How long are your novels supposed to be chapter-wise?
Like, if you only have an hour each day but need at least three hours to finish one chapter, then maybe just focusing on the stockpile for now would be better? Especially if each novel would come out to over a hundred chapters.
As far as audience goes, I think that a consistent release schedule is rather important. Maybe more so than having multiple projects. And the more projects you have, the more difficult it can be to keep releasing consistently. - Take that from somebody who juggled five to six projects at a time which had at least weekly releases if not more with some of them being daily