SimpleX is currently the best one possible.
All the security of signal without needing a phone number.
Everything can be through tor. Contact link can be formed with a one time use code that you DM someone privately.
Anything more advanced and you’re basically in internet dead drop territory. An encoded message on a pastebin through tor. Congratulations, you’ve entered pedophile/terrorist level security realm.
Honestly, pedos & terrorists get the whole world cooperation on doxing them. So if they’re not using a tool or method then it’s not proof against nation states.
Why the need for complete anonymity though? Literally only pedos operate at that level so that nobody can squeal on the others if they are busted (they inevitably need to mess up a real kid because their CSAM isn’t good enough).