consider that caching happens at thousands of levels on the internet. every centralized site has its content replicated many many times in geo local caches, proxies and even local browsers. caching is a very core concept for the internet. others often bash AP because it replicates a lot, but that’s kind of like explicit caching: if the whole fediverse network fetched a post from it source, millions of requests would beat small servers down constantly. big servers cache the content they intend to distribute and handle the traffic spike instead of the small instance. small instances on their hand dont need to replicate as much and can rely more on bigger instances, maybe cleaning their cached content often and refetching when necessary. replication is a feature, not a design flaw!
if you’re trying to build a chatroom then any chat software goes but if you’re trying to build a community you should probably use something searchable and indexed, like real community software
also i find it laughable that users must already be on such platform, by your logic all communities should be mailing lists