Mattermost is the most obvious option; it’s a clone of Slack. IRC is another good option, although I know a lot of people hate it because they prefer features to freedom. I cannot recommend Matrix; the UX is fine but the cryptography has a few issues, as documented by Soatok here.
I’m genuinely curious what would be a good alternative to Discord for something like this
Mattermost is the most obvious option; it’s a clone of Slack. IRC is another good option, although I know a lot of people hate it because they prefer features to freedom. I cannot recommend Matrix; the UX is fine but the cryptography has a few issues, as documented by Soatok here.