Unfortunately a different messaging app or encryption protocol wouldn‘t solve the fundamental problem Signal faces.

Even a Nostr based app would require trusting the app‘s vendor to build the correct source code on iOS.

Until Apple adds support for reproducible builds to iOS, the only way to ensure E2EE for any app’s user base would be to constrain usage to Android.

Jon Callas (co-founder of PGP Corp and Silent Circle) wrote a great post about this in his Reply to Zooko:

https://www.silentcircle.com/blog/reply-to-zooko/ nostr:note1qd2p235vum5euzw5xj07gylx2v9suyqkuxwdazgzmwe8wqndsn5svkuttm