I've been posted here once regarding the possibility to improve accessibility of Apple's devices by unlocking the Siri voices for 3rd party apps.
Meanwhile two things have happened.
First, Apple has invested even more in improving Siri voices in iOS 15 and macOS 12.0 and nothing in other voices.
Second, my friend that is very active in the various fields of tech accessibility took an effort to start the petition for this on the change.org and made a very good explanatory video and text. It seems that this petition is catching up and may grow to the level that it becomes notable to Apple. So if you are about to support the cause of making the world a bit more accessible this is the link to check it and sign it if you agree: https://www.change.org/p/apple-apple-please-allow-3rd-party-apps-to-use-siri-voices-for-improved-accessibility