What about Signal? has e2e encryption, group messaging and is practically completely open source.

I personally really want to quit all the Facebook apps (because of the reasons mentioned), but I just feel that there is too much communication I need in it's messaging apps to leave (family, friends, work chats) that leaving them would be problematic.

What I really want is a messaging app that can still receive and communicate messages from the "bad" apps, but by default uses its better way, so that as people migrate to it, you can communicate with them using the app, but still be able to receive (and send) messages using for example WhatsApp for people that haven't changed apps yet.

I've been looking into telegram alternatives for a while. The best one I've found is the Matrix protocol (https://matrix.org), which is sort of like a cross between telegram and slack but it's open-source and you can self-host it. There are several app/webapp clients that you can use with your server, like https://riot.im.

It's got end-to-end encryption by default (although you need to manually enable it in some clients like Riot), Riot's mobile app works well, and supports audio and video calling through your server.

I use iMessage primarily, it has end-to-end encryption and your messages are stored in iCloud. It also integrates wonderfully well with your Mac.

I'm still logged into WhatsApp though, to keep in touch with the ones using Android, but I've switched notifications off so I just go into check as if I'm checking my email.

I wanted to check out Telegram as a WhatsApp replacement but very few people seems to be using it.

Telegram's source code is such a big ball of spaghetti I would feel really sceptical about its reliability (without, admittedly, having used it).