That is a critical mass thing. Reddit 15 years ago didn’t have a thriving pokemon community either. Things grow naturally over time. I think Lemmy is in a good place :)
But, for example, on Reddit there is r/hockey and a sub for each team. On Lemmy those team subs are graveyards, but if you post on c/hockey you might get enough traction to have a conversation. Find the larger community and help grow that first before fracturing to smaller ones.
What would you have done before the internet?