Session is not very decentralized as Tor because it requires imvesting ~$1000 in a token (which is being replaced soon...) to run a node. Tor nodes can be ran by anyone. Session downgraded their encryption based on incorrect statements: https://getsession.org/blog/session-protocol-explained

Anyone can run the SimpleX servers, there's many other public instances you can use instead. IP address is indeed an identifier, but not part of the messaging protocol itself. You always have to expose your IP address to third parties, it's just your choice who sees it (SimpleX servers, VPN, Tor, etc).

Tor is also fully supported with in-app SOCKS proxy support and effectively kill switch for not connecting directly to non-onion servers, as well as .onion server mirrors. They wrote about it here: https://simplex.chat/blog/20240604-simplex-chat-v5.8-private-message-routing-chat-themes.html 

 Interesting info, you've given me a lot to read about. Thanks!