Did exactly this last week for a health app I work on. Creating it is 5 minutes, the empty room is the real problem. What I'm doing: kept it restricted until it has content, wrote a pinned "start here" post with 3 simple rules, and the plan is to seed it with 10 to 15 genuinely useful posts before inviting anyone. Nobody joins a sub with 0 posts. Main traffic will come from being helpful in the bigger related subs first, not from promoting the new one.