I don't want to sound too nihilistic, and not that I have a good alternative to suggest (besides SimpleX, which, unfortunately, in fact is currently centralized too), yet I can't ignore the fact that Radar uses Signal's servers, which require KYC based on phone number.
I believe Radar is not sustainable (unless they at least remove the KYC, based on the utterly insecure cellular networks).
https://youtu.be/wVyu7NB7W6Y?t=843s
https://youtu.be/kV2HDM86XgI?t=1079s
"We kill people based on metadata" (c) Michael Hayden, Ex-NSA/CIA director
It all starts with PR based on the best cryptographic protocols, while still leaving some issue, which leads to metadata-based association and easier possibility to censor. ProtonMail, for example, has an onion-accessible frontend, but specifically not for new account sign-up. I'm sure it's not by accident.