It took 4 days for Mind Cafe to reply this time around. I had already withdrawn my submission and submitted my piece with the Ascent, but they wrote anyway. The same happened with Better Marketing. And with Better Humans it was something like 5 days after submitting.
The only place where I'm always patient is the Ascent. If it takes 10 days to hear back from them, it doensn't mean it's "no". It's hard to wait, though.
As for niche blogging, I don't know...My niche is gardening, it's quite technical, and it doesn't seem to fit with the Medium vibe. The articles are written in such a manner that they answer speciffic Google search questions. As a beginner, I'm worried that having duplicate content might somehow hurt my SEO, although canonical links are supposed to prevent that. I need to do more research on this one.
Also, Medium is a great place to just unleash your creativity, speak your mind, and just be yourself. With a niche blog, you're kind of limited.