eSpeak NG has been made to work on iOS with AVSpeechSynthesisProviderAudioUnit. It is the first testing release, so there will likely be bugs, but it has been made to work with VoiceOver! The TestFlight beta is available here! The discussion is currently at this GitHub issue, but for bugs and further discussion, you should create a new issue on the app's GitHub repository.
Disclaimer: I am not involved or affiliated with this project in any way.