Your dumb phone is much insecure than a smartphone which has GrapheneOS or LineageOS. Your dumb phone certainly lacks secure 4G or 5G communication, cannot use e2e encrypted messaging platforms and cannot update firmware in case of security bugs.

SimpleX is not a Signal fork. It is it’s own protocol, service and app. It just utilizes Signal protocol for encryption like every good e2e encrypted messenger out there.

SimpleX allows anonymous identity, federation between servers and still a good UX.