I keep researching alternatives, and every time I circle back around toward Matrix, despite criticisms. Part of it is a question of what everyone is using. I pretty consistently see that groups who use or used to use irc are now using Matrix additionally or as a replacement.
Part of me would like xmpp to be the best answer, but I've yet to see an implementation that handles public communities well, particularly for anything that functions at all like Discord. Matrix seems to be at least gaining voice/video chat support?