[Marsymars]

This only works as long as the username is active/unchanged. It would probably be better if usernames were never linkable to phone numbers, but if your threat model requires a persistent, non-ephemeral username to remain anonymous when targeted by law enforcement that has access to your telecom records and warrants... that's going to require a pretty high level of opsec.

The UX on usernames in Signal might be non-ideal. It might be helpful to have a toggle that regularly cycles your username if that's important for your threat model.