Peer to peer communications are difficult to combine with mobile phones (at least if you value battery life). There are various messengers out there, but they're incredibly niche and I doubt they'll ever get any decent user bases.
Tox is peer to peer and encrypted, but its UX will probably drive away anyone who wants the ease of use of Signal or WhatsApp.
I think Matrix experimented with the concept of running a server on-device, and that's one of the few alternative chat systems with decent UIs available, but AFAIK that never made it beyond the proof of concept stage.
Veilid Chat, developed by the Cult of the Dead Cow, promises to be an interesting option, but it's currently in beta and has been for a while.