Disable manual login, and only allow Discord logon

Yes, you can go to admin > customize > site texts and search for the text you’d like to change.

It’s possible with a theme component. This is kind of rough and untested (and I’m pretty sure there’s probably a better way to do it)… but something like this might work? (add to admin > customize > themes > edit CSS/HTML > header)

<script type="text/discourse-plugin" version="0.8.13">
  let currentLocale = I18n.currentLocale(); 
  I18n.translations[currentLocale].js.discord_button = "Log in with Discord";
  
api.reopenWidget("header-buttons", {

  html(attrs) {
    if (this.currentUser) {
      return;
    }
    const buttons = [];

    buttons.push(
        this.attach("link", {
          href: "/auth/discord",
          label: "discord_button",
          className: "btn btn-primary"
        })
    );
    return buttons;
  }
});
</script>