This is confounded by both a) you join all channels on a server, 2) the ability of individuals to 'mute' servers or channels; combined it means it fills up with a bunch of idlers in a way which is worse than IRC as it’s unlikely they will ever read the contents or participate beyond asking a question then leaving.

Right, and this isn't just a minor issue, it is fundamental to Discord and it has serious consequences.

Somewhat astonishingly microblogs like Mastodon or Twitter are generally a more successful place for expert conversations to happen than Discord communities. Think about the amount of Twitter threads written by someone who is an expert in a topic speaking candidly that have been shared with you before whether it was in the context of social justice, your favorite hobby or science. This is because the way conversations sort themselves on Twitter isn't through rigid subchannel structures maintained by topic gatekeepers, conversations are instead "kept on topic" by users having profile descriptions that describe what on topic is for them both in subject matter and tone. Users can then choose to follow or not based upon profile descriptions and previous posts. This provides the necessary "fuzziness" to topic and community boundaries that is required for novel, expert, interesting conversations to happen (though of course microblogs have plenty of drawbacks).

One could easily say that "well, the point of Discord isn't to do serious stuff like that" but Twitter never set out to be a good place for expert, technical discussions. Make no mistake, Twitter made it possible for an activist to write a post describing an unfolding political situation in detail from their phone straight onto Twitter and potentially change the course of history when a major news picks it up... SPECIFICALLY so that teenagers could easily share memes with each other and fans could easily keep up with their favorite celebrity bullshit. Even before shitstick mcspacepants bought Twitter, the company if anything actively disliked this subversive, radically democratic potential within its product.

I think it is damning though that with all the structure Discord brings to the table over something like a microblog, most of the time it utterly fails to elevate the conversation along any metric, especially ones relevant to expert and niche topics. Compare that to Lemmy or Reddit, and even after you handwave away the particular differences of structure and goals in as generous as a fashion possible to Discord, the contrast in quality of conversation and knowledge curated is staggering.