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.

Did you even read the article? 0-day vulnerabilities are not unique to WhatsApp. Even Signal can be exploited given it’s complexity.

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.

I agree SimpleX is superior protocol. I use it to share text between my devices. But I’m little bit hesitant to recommend it to friends and family because it is VC funded. Until SimpleX becomes non-profit or alternative implementation of SimpleX protocol starts showing up, I won’t use it as my main IM.