I’d go for Matrix, with Element or Schildichat (not the next gen versions for now) as clients. It’s federated, which means you can choose which instance you want your account to reside on, end to end encrypted, has (video) calls, and some other features you wouldn’t necessarily expect from others like spaces for groups with several group chats, prioritizing conversations (low, normal, high / favorite) and threads inside a conversation.

Why does it look like Signal might introduce backdoors?

SimpleX-Chat. It’s not as streamlined as Signal for onboarding new users, aka convincing your friend and families. But most things work fine, chatting, calling and filesharing. It’s completely anonymous, as in that you don’t need to register with a phonenumber and or username. You can also self-host and use your own server if you are into that.

The more users it gets, the more investment it will have and become better. A fellow lemming also recommended Briar, but not familiar with it. Privacy enthusiasts also enjoy Matrix so also check that out.

Olvid (French app) is a solid choice. Just so you know, you’ll need a monthly membership to access features like calls and desktop pc app sync.

Try XMPP with snikket, they’re on mastodon too.
Or self-host it.

Personally using Olvid with my friends and family but I guess it’s only for very close connexions. Although I like it lot I doubt anyone else (more distant friends?) will download this app to contact me.

(It’s free without the ability to initiate calls but everything else is here and works perfectly: groupchats, voice messages, sending photos)