I recently started beta-testing Beeper. I was on the hunt/wait for a nice to use client that would be finally able to get all my messengers into a single window for a very very long time (when all my massaging was through XMPP, ICQ and Skype I used to use Trillian), and it is almost it: Telrgram, WhatsApp, SMS and Discord DMs/chat channels in a single app/window.
Keep in mind I value convinience over privacy, but still am mindful about what I send where. Beeper server is not open source, but their bridges are (and encrypt messages before sending to server), and you are able to self host them, I saw in community chat room that some people do that for Signal, so everything is E2E encrypted on hardware under user control.
Signal
Element as matrix client. It has some bugs and glitches but is the current best matrix client and the matrix protocol is the best communication protocol i know.
The protocol is open and well documented, it is fast, you can transmit everything from Voice over IP to IoT, it works decentralised and supports end-to-end encryption.
https://spec.matrix.org/latest/ Here ist the specification website if you want to read more
Signal for private and Whatsapp for work.