If you’re willing I have found that regularly contributing to a community, even if it’s just you, will lead to others participating sooner than you may expect.

That advice about broader topics is also good however. Niche communities aren’t generally necessary until the broader ones become too flooded.

Edit: unless focused on a hyper-niche, with tight focus, such as !obscureinstruments@sh.itjust.works or !soundfontcovers@sh.itjust.works

(Disclosure: I created both examples.