In gaming circles, Matrix is to Discord as Lemmy is to Reddit: tiny. You’re unlikely to find well-established rooms for niche topics, so you would have to either join an existing barely-used room, or start a new one yourself.

The good news is that, with so many people leaving Discord right now, promoting a small room could easily multiply its population and boost its activity. You might even consider talking to moderators of niche Discord channels to see if they’re interested in migrating with you.