Snikket
Snikket is different. The Snikket network is instead made of lots of smaller independent services that can all communicate with each other. Together they form a large messaging network with nobody in the middle. Every person in the network can choose to use a Snikket service provided by someone they trust - this could be an organisation or simply a technical friend or family member!
Snikket is ideal for small groups of people (such as families, friends, clubs or even small organisations) who need a private messaging app that doesn't depend on their phone number, doesn't upload their address book anywhere, and doesn't sell data to advertisers.
self-hosting instructions: https://snikket.org/service/
built on XMPP
I have seen a few recommendations for monal as a client for it, it's a general purpose XMPP client