Sorry to reply to this so late, I procrastinated because unfortunately my answer is that I don't know of any communities, perhaps because I'm a scientist who loves maths rather than a mathematician.

However, I will use this opportunity to share some fun stuff from people I like.

https://youtu.be/H0Ek86IH-3Y by Oliver Lugg on Youtube is great. His channel is very eclectic though, and there isn't much pure maths. I love his shitposting tone though, and he has a discord community that were pretty mathsy when I was in it.

A blog-type site that I enjoy is Tai-Danae Bradley's https://www.math3ma.com/about, largely because I've discovered many other cool researchers through her site.

I also really enjoy Eugenia Cheng's books, especially as someone who is interested in understanding how to write good scientific communication that is accessible without "dumbing things down". I recently finished "The Joy of Abstraction".

Apologies that this isn't what you actually were looking for. I share your distaste at Reddit: I have used Reddit occasionally for those niche communities that aren't available elsewhere (yet!), but the atmosphere is increasingly toxic. I fear that smaller communities that flee are congealing in harder to discover places, like Discord.