To match SimpleX, you need to have the same encryption (Signal Double Ratchet Algorithm) + post-quantim ratchets (like Apple & SimpleX have done https://security.apple.com/blog/imessage-pq3/

https://simplex.chat/blog/20240314-simplex-chat-v5-6-quantum-resistance-signal-double-ratchet-algorithm.html )

As well as using pairwise identifiers for messaging queues (separate sending receiving servers and address) which should be done automatically, behind the scenes so users don't have to create a separate identity for each contact.