Don't know about Nostr based alternative. Would like to know. nostr:npub155m2k8ml8sqn8w4dhh689vdv0t2twa8dgvkpnzfggxf4wfughjsq2cdcvg uses Mattermost. Also Matrix with Element or Commet is pretty similar but encrypted and less centralized. But centralized is usually not a problem for communities.

Check out Flotilla

hivetalk.org

cornychat.com