The only niche tech subreddit I know of that's anywhere near the level of discourse on HN is r/zfs, and even then, it's mostly people asking for help because they broke their zpool; it's just that the answers are often quite detailed in their explanation. r/sysadmin is Windows-centric but isn't awful in terms of depth of knowledge. r/kubernetes is mostly blog spam and "can you troubleshoot <problem I've put zero effort into solving> for me," and r/devops is mostly "how do I get a job in devops."
