Yes, it's 34k websites, but to be transparent, they probably don't all strictly qualify as niche, for example, it includes some smaller commercial sites (usually if they have tech blogs). I've found the hardest part of this isn't finding the feeds so much as filtering out the usual suspects (SEO etc) that masquerade as technical content.

As well as HN, I found a google search along the lines of "site:github.com opml rss feed [topic]" was quite effective too.