It’s a shame that iOS limits the use of Siri voices. Considering all other voices, be it premium or enhanced, kinda suck, the best option for TTS would be Apple’s Personal Voice.
It seems that now we are allowed to use Personal Voice in third-party apps, as long as the app requests it, e.g. Speech Central, an app for general TTS purposes.
It would be hugely appreciated if this were possible with AnkiMobile as well!