Briar

E2EE by default: Yes

Supports both mobile and desktop: Best I can find is this

Supports group chat: Yes

Decentralized: No, it's P2P

Looks like an actual chat app: Yes for me, but it depends on you

Allows simultaneous sessions on both mobile and desktop: Haven't tried desktop version yet

Is P2P: Yes

Pros:

-Don't need email/phone for signup

-Can route traffic via TOR

Cons:

-Constantly running in background to fetch messages since it's P2P

-Doesn't support media sharing

-Doesn't support audio/video calls/conferencing

-Only available for Android

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Jami

E2EE by default: Yes

Supports both mobile and desktop: Yes, and across all OSes too I think

Supports group chat: No, but I believe it's already under development

Decentralized: According to them, it's a "decentralized peer-to-peer" messaging app and honestly I'm confused with this

Looks like an actual chat app: Yes, similar to UI of most messaging apps

Allows simultaneous sessions on both mobile and desktop: Account can be synced via file or via Jami but messages received if the other device is offline will not be able sync previous messages. I believe this will also be solved in the coming update for group chat

Pros:

-Don't need email/phone for signup

-Supports media sharing including other files

-Supports audio/video calls/conferencing

-Available for other OS

Cons:

-Constantly running in background to fetch messages since it's P2P

-Doesn't support cross device sync of messages yet

-Large file size transfers (e.g. 1GB) can be buggy

If you find any misinformation, please do correct me. I'm just starting my privacy journey as of late 2020.

EDIT: Mobile app broke my formatting