While I don’t personally know any good servers for those topics, https://disboard.org is a good resource for finding new servers that match your interests. Good luck!
Any advice for finding quality Discord servers (things like ideal member count / “red flags” etc)? I feel like I have a lot of trouble finding servers that strike a good balance of being active but not impenetrable.
I don’t have any, other than maybe look in general chat to make sure it’s active. I’ve also struggled with this, often a promising new server is disappointing after I get to know it over a couple days.
It's a mess of malignant attention whores and child predators. The servers that are good inevitably attract the bad types who, when removed, go out of their on way to take the server down. Don't waste your time.