Based on what I understand about your use case, I'd suggest considering the following if looking to improve the security/privacy posture.
- A Hash style ratchet still works in your decentralized construction; Users ratchet a lamport timestamp, hash ratchet the next key. It still allows multiple messages to be sent with the same key if messages are sent simultaneously, but it's better than the approach where every message uses the same key. Ref: B
- There is real work on making Decentralized MLS work without requiring classic servers: (Disclosure: Not my work, but affiliated) https://lip.logos.co/anoncomms/raw/decentralized-mls-offchai.... It's far from ready, and It definitely adds more complexity, but provides the same server-less architecture without sacrificing on security. Ref: A+B+C
- Push not Pull: Query based interfaces are hard to protect from unmasking attacks. PubSub based peer to peer networks decouple the intended recipient from a message - Messages are broadcast which provides the necessary ambiguity about who the intended recipient is. Combined with P2P syncing for message durability. Ref: D
- A Hash style ratchet still works in your decentralized construction; Users ratchet a lamport timestamp, hash ratchet the next key. It still allows multiple messages to be sent with the same key if messages are sent simultaneously, but it's better than the approach where every message uses the same key. Ref: B
- There is real work on making Decentralized MLS work without requiring classic servers: (Disclosure: Not my work, but affiliated) https://lip.logos.co/anoncomms/raw/decentralized-mls-offchai.... It's far from ready, and It definitely adds more complexity, but provides the same server-less architecture without sacrificing on security. Ref: A+B+C
- Push not Pull: Query based interfaces are hard to protect from unmasking attacks. PubSub based peer to peer networks decouple the intended recipient from a message - Messages are broadcast which provides the necessary ambiguity about who the intended recipient is. Combined with P2P syncing for message durability. Ref: D
As you said, its all tradeoffs ¯\_(ツ)_/¯