Signal requires that you share a telephone number. SimpleX only requires that you share an address identifier, which is not tied to you in any way. And you can use separate addresses for separate people so that you cannot be identified if you’re talking with two buddies. It provides a more anonymous alternative to Signal in a format that approximates Telegram but which has greater security than Telegram.

Nostr is not appropriate for small group chats, and it’s not anonymous (relays can see IP’s if they do desire).