XMPP is an extensible protocol that has over a decade of battle testing from the casual chat to massive industrial communications applications (Zoom, Jitsi, almost certainly any online game you’ve played). It has E2EE in modern clients. It’s decentralized by nature & relatively easy to self-host. Both servers & clients use very few resources like bandwidth, storage, processing, memory (consider conditions of the time of invention). It doesn’t take minutes to join & sync chatrooms (MUCs). Gateways allow folks to talk across non-XMPP platforms. Governance is distributed in the open & not tied to a single entity. There are even projects like Snikket that can be rolled out for a family that is close to turn-key for set up. Along with something like Movim can create a self-hosted social network built atop an XMPP server for posts to share stories & media for a longer-term storage.

If E2EE encryption isn’t seen as a must relying on TLS + self-hosting: lighter, simpler IRC (good feature set with v3) which has been around since the ’80s can be a good choice. Zulip which is a forum/chat platform that has the most usable UX for trying to actually hybridize both (it’s not amazing UX, but better than the rest); this can work for a great for certain communities that desire this behavior.

Distributed (not to be confused with decentralized) encrypted chat there is Briar with a mesh network not even requiring internet, but has limited platform support & last I used years ago had massive battery drain issues.

If you must, there is Matrix which decentralized & offers E2EE but is relatively expensive to run from the clients, to servers, to the design generally being that it replicates the room messages & attachments & state across all servers for all users. While that duplicated data is great for resilience, can be expensive to store & is what takes minutes to join any room. I think it was a design decision ‘miss’ to try copy Slack/Discord/Telegram-but-FOSS as doing too much & none of it that well–where I think chat is better to be a bit simpler + expected to be ephemeral & a different service like a forum for important, permanent discussions & FAQs. Mastodon suffers similar issues with replication that makes some have to shutdown their self-host due to cost–which has led to Matrix in practice centralizing around Matrix[dot]org (who has a history of Israeli intelligence funding) & the servers they provide to others funneling all the metadata thru their org since they offer free accounts, are big enough to scale, & have most of the users. Folks act like Matrix is great just for being newer, but the aforementioned already cover its uses while being more mature.