I'm a middle school teacher who knows all of the high schoolers as well from my town where the adults certainly aren't anime people (mostly blue collar workers and lots of migrants as well), and there isn't a single kid in my school who doesn't know what demon slayer is at least and who couldn't tell you who the two main characters are, even if they don't watch it or anime in general. I feel like the youth would show up for sure (and I personally would die of happiness)
I'm not saying that makes it mainstream as opposed to niche, just food for thought. I personally don't know that it falls into either category, more "mid" as the kids say in terms of exposure.