I don't really see Signal as being any better than WhatsApp. Technically, Signal is open source, but it's still all controlled by one company and forks cannot interact with users on the official apps. If the company behind Signal decided one day to add a backdoor or remove the E2E there is 100% nothing you can do as a user to stop it.
This is pretty much the reason that caused me to stop pushing for Signal. That and the fact that I can't get it on f-droid.
@kiri Yup, and there is no technical reason that it can't be on F-Droid. It's just that the OWS doesn't want to publish an official version there.
Someone could build a unofficial client but it would not be able to communicate with any Signal users using the official apps. Plus if they used any Signal branding it would get DMCAed like LibreSignal was.
I used telegram (available on f/g-droid) until i set up my own XMPP server running Prosody (and Jitsi for voice calling) using this (https://samhobbs.co.uk/2016/09/installing-prosody-instant-message-chat-server-raspberry-pi-ubuntu) guide.
I used a raspberry pi (received as a gift) but wpuld now recommend this (https://libre.computer/products/boards/all-h3-cc/) libre computer board (which i've purchased but not tested yet as it's in the post!).
piira.org is one server in the network