The user did not respond to any questions, but developing such applications is only used by a small group that is interested in privacy, and therefore it is difficult for them to trust your application.
I can't find other tools on Android that can do this it seems only Openkeychain app is the only app available I don't know for IOS it seems it does not yet exist.
You can try
https://keybase.io. It is open source and available on IOS and has end-to-end encrypted messages. IOS is a closed source operating system and they have back doors, so using it for privacy may not be a suitable option.
https://apps.apple.com/gb/app/keybase-crypto-for-everyone/id1044461770?l=cs